THE DEVELOPMENT OF MAMMALS 547 



264. Growth of the Embryo 



The main morphologic changes in embryonic development take 

 place surprisingly fast. A human embryo four weeks old is only 5 mm. 

 long, but it has already developed enough to be recognized as some sort 

 of a vertebrate embryo (Fig. 31.7 A). The development of all of the 

 organ systems is well under way, the heart has begun to beat, limb buds 

 that will differentiate into arms and legs are protruding from the surface, 

 and a small tail is present. Pregnancy may only be suspected at this 

 time. At eight weeks (Fig. 31.7 B), the embryo can be recognized as 

 human. The face and distinct fingers and toes have developed. The organ 

 systems are approaching their adult condition. Some of the bones are 

 beginning to ossify and taste buds are developing on the tongue. The 

 embryo is arbitrarily called a fetus from this age on. 



Only relatively small changes occur in the organ systems during the 

 remaining seven months of pregnancy, but a great increase in size takes 

 place. An eight-week fetus has a crown-rump length of 30 mm. At term, 

 its crown-rump length is about 35 cm. Among the morphologic changes 

 that occur during this period are differentiation of the external genitalia, 

 development of body hair, muscularization of the digestive tract, and 

 myelinization of the neurons. Though the infant is well developed at the 

 time of birth, development does not cease. Changes in the organ systems 

 and in the relative size of body parts continue throughout infancy, child- 

 hood and adolescence. Human development is not really completed until 

 the late teens. 



265. Twinning 



Many offspring are born at the same time in pigs, rats and a number 

 of other mammals. The number in a pig litter, for example, ranges 

 from 7 to 23. But many other mammals, including man and the other 

 higher primates, whales and horses, normally have only one offspring 

 at a time. Occasionally multiple births occur in these mammals. Twins 

 are produced about once in every 88 human births. Approximately three- 

 fourths of these are dizygotic, or fraternal twins. Two eggs have been 

 ovulated and fertilized at about the same time. Such twins do not 

 resemble each other any more closely than brothers or sisters born at 

 different times, for they have somewhat different genetic constitutions. 

 Fraternal twins occur more frequently in some families than in others, 

 so it is possible that there are certain hereditary tendencies for the 

 maturation and ovulation of more than one ovum during a single 

 menstrual cycle. 



More rarely, monozygotic or identical twins are formed. Only one 

 egg is fertilized, but two embryos develop from it. Identical twins are 

 always of the same sex and resemble each other closely for they have 

 identical genetic constitutions. Monozygotic twinning may occur in one 

 of several ways. The two blastomeres produced by the first cleavage may 

 separate and each become an embryo, the inner cell mass may subdivide, 

 or two primitive streaks may develop upon a single embryonic disc. 



