EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 55 



In examining the eggs of reptiles and birds we note that they are en- 

 closed within leathery or brittle shells. Within these shells must be stored 

 enough food, yolk, to nourish the embryo until it is ready to hatch and 

 begin actively foraging for its food. Everyone is familiar with the large, 

 yellow, globular yolk mass in a hen's egg, for example. The embryo de- 

 velops on top of this mass, and early in its development encloses the yolk 

 in a large yolk sac (Fig. 8.24, p. 169). All birds and reptiles do the same. 

 Although mammalian embryos obtain their nourishment in an entirely dif- 

 ferent manner, nevertheless they develop yolk sacs connected to the di- 

 gestive tract just as do reptile and bird embryos (Figs. 4.4, 4.9). Why? As 

 in the cases of rudimentary or vestigial organs possessed by adults (pp. 

 40-44), the most reasonable explanation seems to be that mammals in- 

 herited these useless sacs from ancestors for which yolk sacs were func-> 

 tional organs. To be specific, it is believed that mammals inherited theii 

 yolk sacs from reptiles, since there are many reasons for regarding reptiles 

 as ancestral to mammals. We may note in passing that mammals also in- 

 herited in greatly reduced form another embryonic structure of great 

 value to reptile and bird embryos, the allantois (see pp. 169-170). 



Somites in Human Embryos 



On both sides of the neural tube somites develop (Fig. 4.5C and D). 

 These are more or less cubical blocks of mesodermal tissue forming be- 

 tween the ectoderm and the endoderm; the ectoderm is molded over them 

 so that their outlines are visible externally. The first ones form just pos- 

 terior to what will be the head. Subsequently the number is increased by 

 formation of somites posterior to these first ones (Fig. 4.5). Thus a row 

 of somites arises on each side of the central nervous system. 



Somites form, among other things, the beginning of the muscular sys- 

 tem. Most of the muscles which later attach to the skeleton and make pos- 

 sible movements under conscious control (voluntary, striated muscles) de- 

 velop from these somites, directly or indirectly. The adult muscles have 

 the greatest diversity of size and shape, and utter lack of resemblance to 

 the rows of blocklike somites from which they came. Such disparity be- 

 tween embryonic beginnings and adult structure clearly calls for explana- 

 tion. 



Since the muscular systems of all vertebrate embryos agree in beginning 

 as rows of somites, this pattern is evidently an ancient one. The most 

 primitive vertebrates, and those which we learn from the fossil record 

 were first to appear, are the fishes. Fish embryos, like embryos of other 

 vertebrates, develop rows of somites. Subsequently these somites develop 



