GASTKULATION 



33 



The fact that the yolk portion of the blast u la. consists not of 

 a single layer of cells as in Amphioxus but of a solid bulky mass 

 forming a large proportion of the whole blastula, renders it physically 

 impossible for the yolk hemisphere to be involuted bodily into the 

 interior of the apical 



hemisphere. As a A F 



consequence \vc find 

 in Polypterus that 

 the process of in- 

 volution is replaced 

 to a -ivatt-r extent 

 than in Amphioxus 

 by overgrowth, the 

 gastrular lip grow- 

 iiiL 1 , over the mass 

 of yolk-cells as seen 

 in Fig. 19,C,D and 

 E. As this process 

 of overgrowth con- 

 tinues the project- 

 ing yolk-plug the 

 mass of yolk - cells 

 not yet enclosed 

 gradually dimin- 

 ishes in size and 

 eventually disap- 

 pears completely in 

 the now narrow 

 blastopore. As yet 

 material is not avail- 

 able to show defin- 

 itely whether the 

 overgrowth is more* 

 active in what corre- 

 sponds to the an- 

 terior portion of the; 

 gastrular lip of 

 Amphioxus but the 

 probability is in 

 favour of this being 

 the case anc^ the 

 figures 



y.p. 



Km. T.. Illustrating jivore.ss of gastruliitiou ill 7W/////V/-/'.-,-. 

 (Ki-s. A, C, E, F after drawings by Budgett.) 



are onen- 



'/./, gutrttlax lip; .</./', yolk-pluj,'. The e<;g is in each case viewed 

 from the left side and has the dorsal side above. The original apical 

 pole is directed downwards anil towards the left side of the page as in 

 tated On the aSSUmp- the preceding figure of A ///./ to t*& 



tion that this is so. 



The two features to be specially noted in the gastrulation of 

 Polypterus as compared with that of Amphioxus are (l)ithe 

 accentuation of the process of overgrowth and the reduction of I the 

 process of involution and (2) the tendency, in early stages at least, 



VOL. II D 



