INFLUENCE OF FOOD YOLK 271 



with food-yolk, and we accordingly find that the mammalian 

 ovum (Fig. 71) has become reduced in size again until it is 

 no larger than that of Amphioxus, i.e. about 2so tn i ncn m 

 diameter. The early stages of segmentation have also gone 

 back to the primitive type, but, strange to say, a yolk-sac is still 

 formed although there is no yolk for it to contain affording an 

 admirable example of a vestigial embryonic organ. Here one 

 record has been superposed upon another in the ontogeny and 

 we have a recapitulation of an embryonic stage of the reptilian 

 ancestors of the mammals. After birth, of course, the young 



FIG. 120. Embryo of a DogBsh, with Yolk-Sac (Y.S.) attached, X 2. (From 



a photograph.) 



mammal is nourished by the secretion of milk, and is thereby 

 enabled to reach a very advanced stage of development before it 

 has to begin to find its own food. The chick, owing to the very 

 large amount of food-yolk with which the egg is supplied, is also 

 able to reach a highly developed condition before it hatches and 

 begins to look after itself. 



With animals whose eggs contain insufficient food-yolk, and 

 which are not provided with any other special means of 

 nourishing the developing young, the case is very different. They 

 are obliged to hatch and begin life on their own account while 

 still a long way from the adult condition when only a small 

 part of their ontogenetic journey has been accomplished. 

 Amphioxus, for example, hatches as a free-swimming ciliated 

 embryo shortly after the gastrula stage has been passed through, 

 and completes its development at the expense of food supplies 

 which it obtains for itself. 



