308 C, O. WHITMAN. 



lengthens backwards, like the embryo of the fish or of Ciepsine. 

 This is also in harmony with the latest investigations of His, who 

 admits that the bird passes through a stage comparable with the 

 Gastrula of other animals (y^)' 



The cause of the central thinning of the blastodisc, the 

 direction of growth, and the shape of the band-curves assumed 

 at successive stages, are all quite clear in the case of Ciepsine. 

 Is the thinning of the central field to be explained in the same 

 way in the case of the bird and the fish ? Since the process in 

 both cases leads to similar results, it is natural to infer that it is 

 controlled by the same general laws. Fig. y represents a diagram- 

 matic section of Ciepsine at a little earlier stage than that of 



The cells of the blastodisc are rapidly multiplying by divi- 

 sion and lengthening as they divide. The consequent expansion, 

 due not only, as before explained, to the multipUcatiou of the cells 

 of the central field, but also to ihe addition of cells to the 

 germ-bands from behind, disturbs the equilibrium of pressure. 

 The effect of the increased pressure at the margin of the disc on 

 the underlying yolk manifests itself in the downward movement 

 of the lateral blastomeres (a and L) and the upward movement 

 of the central blastomere {c). Suppose the yolk, instead of 

 being divided into three segments, to be a single mass as in 

 the egg of the fish or the bird ; the pressure exerted would 

 still be in the same direction and would generate movements in 

 the yolk similar to those represented in the figure. ^If the resis- 

 tance offered by the yolk were great it might cause the disc to 

 arch against the vitelline membrane and thus produce an 

 embryonic cavity ( Keimhohle). The arch would, however, be 

 opposed by the membrane and thus the pressure at the margin 

 would overcome the resistance of the yolk. The equilibrium of 

 pressure would tend to re-establish itself by movements equi- 

 valent to those indicated by the arrows. The downward pressure 

 at the periphery of the yolk would cause the central yolk to take 

 the direction of least resistance — upward. 



The comparison above sketched may, I believe, be extended 

 to the formation of the entoderm. 



In Ciepsine the ectoderm develops from one pole of the egg ; 

 the primary entoderm consists of three large blastomeres {a, b, c) 

 on which the ectoderm rests. The entodermic spheres are so 

 loaded with nutritive material that a regular cleavage cannot take 

 place. The consequence is a free formation of nuclei by the 

 repeated division of the original nuclei. These nuclei, surrounded 

 by a little protoplasm, take a perijiheral position (fig. 9, w), and 

 later, by a sort of superficial cleavage, appear as cells of the 

 secondary (permanent) entoderm. The cpibolic expansion of the 

 ectoderm with the mesodermic elements causes a solid invagiuation 



