64 



COMPARATIVE ANATOMY 



produced within this growth zone or "germ ring" are added, some to the 

 outer layer and some to the inner layer. This growth process, then, is 

 concerned both in the narrowing of the blastopore and the elongating of 

 the embryo. A direct consequence of it is that the material of a certain 

 region of the inner layer immediately adjoining the blastopore attained 

 its internal position not as a result of the primary invagination but by the 

 secondary growth process. 



ANIMAL POLE 

 I 



VEGETAL POLE 



ECx 



Fig. 42. — Gastrulation in AMPHIOXUS. The figures represent sections through 

 the polar axis of the embryo. A, blastula with vegetal region flattened; £ and C, earlier 

 and later stages of invagination of vegetal hemisphere; D, gastrulation completed; with 

 elongation of the gastrula, its long axis becomes the horizontal antero-posterior axis 

 of the embryo. A, archenteron; B, blastocoele; BP, blastopore; EC, ectoderm; EN, 

 endoderm; P, polar body. (After Cerfontaine.) 



At the close of the gastrula period (Fig. 4.2D) the embryo is an elongated 

 ovoid, the slightly larger end being anterior while the now very narrow 

 blastopore marks the posterior end of the lang axis. So rapid is develop- 

 ment that this stage is attained about seven hours after fertilization. 



Significance of the Gastrula. This gastrula form is of utmost signifi- 

 cance, certainly prospectively, and perhaps also retrospectively. Reduced 

 to lowest terms, what, essentially, is an organism? It is something which 

 carries on perpetual interchange of material, nutritive and respiratory, 

 with the environment, and it is capable of making such adjustments with 



