438 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



in the fact that here as in the Dipnoi the embryonic body during 

 the early stages of its differentiation has a much greater angular 

 extent, curving round the mass of yolk cells instead of being 

 restricted to a small extent near the apical pole. Another import- 

 ant point to notice is the well-marked downward flexure of the 

 head during early stages — a feature which has already been corre- 

 lated (p. 93) with the presence of a large supply of yolk. The 

 active forward growth of the head-region leads to the rounded 

 main mass of yolk being situated well back, just in front of the 

 anal region (Fig. 202, C), instead of anteriorly as in the tadpole- 

 shaped larvae of Lepidosiren or Protopterus or the Ganoid fishes. 



Elasmobranchii. — Of the isolated groups of Vertebrates char- 

 acterized by having meroblastic eggs the most nearly primitive is 

 that of the Elasmobranch fishes. Unfortunately for purposes of 

 comparison we are not, up to the present, acquainted with any 

 member of the group possessing small eggs poor in yolk. 



Here as in other groups with typically meroblastic eggs the 

 early rudiment of the body of the embryo — or more correctly of 

 the dorsal portion of the body — extends through a relatively small 

 angular extent, in striking contrast with the 200° or more of a 

 Lung-fish or of one of the Gymnophiona. As the embryo proceeds 

 with its development it grows actively in length, headwards and 

 tailwards, so as to project freely in a tangential direction, remaining 

 in connexion with the main mass of the egg (yolk-sac) by a narrow 

 yolk-stalk. During the forward growth of the head growth-activity ' 

 is less pronounced on the ventral side so that the gill -clefts are 

 forced into an oblique position and the head undergoes pronounced 

 cerebral flexure. 



Amniota. — In the non-mammalian Amnio ta 1 the first point to 

 notice is that although the size of the egg and the absolute amount 

 of food -yolk contained within it are relatively enormous yet the 

 degree of telolecithality is less extreme than it is in the case of the 

 meroblastic eggs of fishes. Consequently the segmentation process 

 resulting in blastoderm formation spreads in an abapical direction 

 past the level at which the posterior end of the embryonic rudiment 

 will be developed : as a result of this the entire embryonic rudiment 

 lies at its first appearance well within the boundary of the blastoderm, 

 instead of its hinder end being coincident with that boundary as 

 is the case in Elasmobranchs. 



Here again the embryonic body grows forwards and backwards 

 free from the surface of the egg, the backward growth being much 

 less pronounced than in the case of the fishes in anticipation of the 

 ultimate lesser degree of development of the tail-region correlated 

 with its diminished locomotor importance in the adult. 



The most striking feature however is one which may perhaps 

 be correctly expressed by saying that the clogging influence of the 



1 The external features in their development are well illustrated by the developing 

 Bird (see the figures in Chap. X.). 



