FATE OF BLASTOPORE, ETC., IN CHELOXIA. 3 



thus bending downwards, a mass of cells without any regular 

 arrano-ement rises upwards until it reaches the level of the general 

 surface of the embryonic shield. This is the section of the mass 

 above referred to. In the first of the present series of Contribu- 

 tions,* ISHIKAWA and I gave a minute description of this mass, 

 as it occurs in Trionux, and from its relations to the adjacent parts 

 homologised it with the yolk-plug of the amphibian ovum. We made 

 this comparison independently of any previous writer and, believed 

 ourselves to be the first to |>oint out the existence of a yolk-plug in a 

 meroblastic amniote ovum. I have since found that previously to our 

 publication Duval ('84) had discovered a similar structure in the 

 nightingale and in the fowl (loc. cit. figs. 15 & 32), and had enunciat- 

 ed a hypothesis which, although not exactly like ours, is of similar 

 tenor. If I understand him aright, he did not directly homologise 

 the mass sticking out between the lips of the blastopore (his fig. 15, 

 PJ^) with the yolk-plug of the Amphibia, but called certain cells, 

 arisen by secondary segmentation and resting on the mass (his fig. 

 10, n & fj), the (jlohuks de Ecler as he considered them homologous 

 with the hotichon de Ecker or yolk-plug {y. 68). 



Since our publication, Van Beneden ('88) seems to have ob- 

 served a similar structure in Mammalia and regarded it in the same 

 light as ourselves. He says : — " Der Boden des (Chorda-) Kanals, 

 ist mehrsichtig und ist nach hinten in Zusammenhang einerseits mit 

 dem Hypoljlast, anderseits mit einer Zell masse, die vom Bo- 

 den der Brimi tivrinne zwischen den Li[)pen dersell)en 

 hinaufsteigt. ^ie entsi)richt offenbar dem Dotter- 



pfropf der Am])hibien und ist olme Zweifel mit dieser 

 For m a t i (j n li o m o 1 o u". ' ' 



* See tlie list ;it the end of this article. 



