GASTKULATION 



33 



The fact that the yolk portion of the blastula 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 ^ | 



consequence we find 

 in Polypterus that 

 the' process of in 

 volution is replaced 

 to a greater extent 

 than in Amphioxus 

 by overgrowth, the 

 gastrular lip 

 ing over the mass 

 of yolk-cells as seen 

 in Kg. 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 c6mpletely 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 and the 

 figures are orien- 



Fig. 19. — Illustrating process of gastrulation in Polypterus. 

 (Figs. A, C, E, F after drawings by Budgett. ) 



ij.l, gastrular lip ; y.p, yolk-plug. The egg is in each case viewed 

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

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

 tated On the aSSUmp- the preceding figure of Amphioms. 



tion that this is so. 



The two features to be specially noted in the gastrulation of 

 Polypterus as compared with that of Amphioxus are (1) the 

 accentuation of the process of overgrowth and the reduction of the 

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



VOL. II D 



