464 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



III. Adaptive Modifications in the Development of the 

 Amniota. — It is characteristic of many Vertebrates that, associated 

 with the provision of special arrangements for nourishing the young 

 individual, the time of commencing an independent life on its own 

 account is greatly delayed. In such cases where a considerable 

 proportion of the whole development takes place within the 'shelter 

 of the egg-shell (or of the parental body) we nave to do with what is 

 known as embryonic in contradistinction to larval development. 

 During embryonic development the young individual is free from the 

 necessity of fighting and fending for itself ; it is to a great extent 

 sheltered from the struggle for existence, and in correlation with 

 this we find remarkable hypertrophies and modifications of various 

 parts of the body taking place which in a free state would render 

 life impossible. 



The first of these modifications makes its appearance in the lower, 

 aquatic, Vertebrates in the form of a pronounced bulging of the 

 ventral side of the body. In the more primitive holoblastic Verte- 

 brates this is caused by the great thickening of the ventral endoderm 

 (Fig. 80, E, p. 146), its cells being much enlarged and packed with 

 granules of yolk. Where this distension of the endoderm cells is most 

 marked anteriorly there is brought about the tadpole shape of body as 

 seen in the Ganoids and Lepidosiren : or, on the other hand, the 

 distended region may be situated .towards the hinder end as in Petro- 

 myzon, Ceratodus or the Gyrnnophiona. In such cases as develop- 

 ment proceeds the large yolk-cells go on segmenting, the yolk within 

 them is gradually used up, and the mass of endoderm, becoming more 

 and more attenuated, ceases to project beyond the general outline of 

 the body. 



In the meroblastic egg, as has already been shown, the proportion 

 of living protoplasm amongst the yolk has been reduced to vanishing 

 point so that except superficially the yolk never segments. Typically 

 it becomes gradually enclosed in the endoderm which spreads over 

 its surface. There is thus formed what is known as the yolk-sac, a 

 structure usually of enormous size as compared with the rest of the 

 embryo. It will readily be understood how impossible a free active 

 existence would be while there is a large yolk-sac present. The 

 assimilation of yolk and its transport to the actively growing parts of 

 the embryo are brought about mainly by the rich development of 

 superficial blood-vessels forming the vitelline network. In typical 

 Teleosts, e.g. Salmonids, the yolk-sac becomes at an early period com- 

 pletely separated from the dorsal part of the endoderm which becomes 

 the functional gut, the yolk absorption taking place entirely by the 

 vitelline vessels. 



An important point to be remembered is that the vitelline net- 

 work though primarily nutritive in function is necessarily also 

 respiratory, gaseous interchange taking place between the blood 

 circulating in its vessels and the medium which bathes its surface. 

 The vitelline network is the primary breathing organ in the great 



