chapter 36

Protists can reproduce asexually by , and simple animals can reproduce by .
- Asexual reproduction.
  The unicellular protist Euglena reproduces asexually by fission: one cell undergoes mitosis, yielding two genetically identical organisms.
  Simple animals such as hydra can reproduce by budding: a part of the animal's body pinches off to produce a clone of the original animal.
  Hydra budding:
Some species exhibit , where one individual contains both male and female organs.
- Hermaphroditism and protogyny.
  The hamlet bass is a hermaphrodite (one individual has both testes and ovaries) and switches sexual roles during mating.
  Here a male-acting fish curls around its partner, fertilizing "her" eggs.
  
  The bluehead wrasse exhibits sequential hermaphroditism, or protogyny: the largest female in a population turns into a male when the dominant male is absent.
Sex in mammals is determined by the presence of the gene on the chromosome.
- Sex determination in mammals is made by the SRY gene located on the Y chromosome.
  Testes are formed when SRY is present; ovaries are formed when it is absent.
  Sex chromosome aneuploidies can lead to abnormal sexual characteristics.
Animals evolved in the where fertilization usually occurs .
- • External fertilization.
  When frogs mate, the clasp (amplexus) of the male induces the female to release her eggs, over which the male discharges his sperm.
  Frogs are poikilothermic (body temperature varies with the environment), and do not incubate the eggs after fertilization.
Birds and mammals undergo fertilization and are homeotherms.
- Temperature homeostasis.
  Birds are homeothermic and need to maintain a constant body temperature.
  This blue-footed booby is incubating its two eggs to keep them warm.
  Boobies courtship:
In human males, sperm is produced in the in a process called .
- A sperm is propelled by a flagellum attached to a body packed with mitochondria that provide energy for swimming. The head contains the nucleus.
  The acrosome contains enzymes that help the sperm penetrate the egg for fertilization.
  Human sperm:
- Male reproductive system
  The testis (held in an extension of the abdomen called the scrotum) is where sperm is formed.
  Sperm complete maturation in the epididymis, and is stored in the vas deferens (ductus deferens).
  Seminal fluid is produced in the seminal vesicle, prostate gland, and bulbourethral gland.
  The seminal fluid mixes with sperm to yield semen and exits via the urethra, which is also the exit duct from the urinary bladder.
  Labeling exercise:
- Spermatogenesis. Germinal cells in the seminiferous tubules of the testis mature into diploid primary spermatocytes, which undergo meiosis (via secondary spermatocytes) to form haploid spermatids. Spermatids develop into spermatozoa (sperm), nurtured by Sertoli cells. Review:
Female gametes develop in the in a process called .
- Female reproductive system
  Eggs are produced in the ovary and released into the Fallopian tube (oviduct) during ovulation.
  Fertilization occurs in the Fallopian tube. The fertilized zygote travels to the uterus, then implants in the uterine wall and develops into an embryo.
  The child is born through the vagina, which is a separate opening from the urethra.
  Labeling exercise:
- Oogenesis. Germinal cells mature into diploid primary oocytes. Every 28 days starting at puberty, one primary oocyte undergoes meiosis I. One of the haploid daughter cells is a nonfunctional polar body; the other (secondary oocyte) grows in a follicle and is released (ovulation). Meiosis II occurs only if the secondary oocyte is fertilized by a sperm, producing an ovum and another polar body. Review: