448 MORGAN. [Vol. X. 



tion throughout the elongation of the embryo. The distance 

 CD, also, does not increase sensibly during the period of the 

 elongation of the embryo. The main increase is in the region 

 BC, lying between Kupffer's vesicle and the anterior end of 

 the notochord. 



The number of protovertebrae increases rapidly. At the 

 first stage about 3 or 4 pairs are present ; at the last stage, 

 about 24 pairs. Fol found in the chick, by marking the blas- 

 toderm, that the first protovertebra retains its position, and 

 the rest form posteriorly. Henneguy's measurements show 

 the same to be true for the fish. Since the anterior pair of 

 protovertebrae retain a constant distance from the anterior end 

 of the notochord ; and since the size of the protovertebrae as 

 they form remains the same ; and since the distance between 

 the last protovertebra and the end of the embryo remains about 

 the same, — it follows that there is a growing region between 

 Kupffer's vesicle and the last protovertebra formed. Here the 

 elongation and, pari passu with the elongation, the cutting off 

 of new parts of protovertebrae takes place. 



Lereboullet (8) described, in 1863, certain abnormal embryos 

 in which the right and left sides of the body were separated 

 from one another by an exposure of yolk.^ The anterior and 

 posterior ends had their right and left sides united so that only 

 the middle portion of the body had its halves separated. At 

 a later stage the halves in the middle region united to one an- 

 other. Lereboullet interpreted this to mean that each half of 

 germ-ring developed in the normal embryo into a corresponding 

 half of the embryo, and in three abnormal embryos the union 

 of the halves had been retarded, and did not take place until 

 the organ had differentiated. Each half ring, therefore, formed 

 its half organs in situ. 



It is clear from Lereboullet's account that he anticipated, 

 by ten years, the conception of the concrescence of the fish 

 embryo, usually ascribed to His. Oellacher (11), in 1873, 

 rejected Lereboullet's theory, and believed that elongation 

 took place " auf das raschere Langswachsthum des Medullar- 

 rohres und auf seinen Bildungsmodus. . . . das Medullarrohr 



1 The protruded yolk was really covered by a thin layer of ectoderm (!). 



