332 F. M. BALFOUR. 



Stages, It appears, however, to have somewhat different re- 

 lations to the blastoderm than the homologous structure in 

 other vertebrates. In its earliest stage which I have observed, 

 it appears as a small cavity in the centre of the lower layer 

 cells. This grows rapidly, and its roof becomes composed 

 of epiblast and only a thin lining of " lower layer" cells, 

 while its floor is formed by the yolk (PI, XIII, fig. 3, 5 c). In 

 the next and third stage (PL XIII, fig, 4, sc) its floor is 

 formed by a thin layer of cells, its roof remaining as before. 

 It has, however, become a less conspicuous formation than 

 it was ; and in the last (fourth) stage in which it can be 

 distinguished it is very inconspicuous, and almost filled up 

 by cells, 



"What I have called the second stage corresponds to a 

 period in which no trace of the embryo is to be seen. In 

 the third stage the embryonic end of the blastoderm projects 

 outwards to form a structure which I shall speak of as the 

 " embryonic rim," and in the fourth and last stage a distinct 

 medullary groove is formed. For a considerable period during 

 the second stage the segmentation cavity remains of about the 

 same size ; during the third stage it begins to be encroached 

 upon and becomes smaller, both absolutely, and relatively to 

 the increased size of the germ. 



The segmentation cavity of the Dog-fish most nearly 

 agrees with that of Osseous fishes in its mode of formation 

 and relation to the embryo. 



Dog-fish resemble Osseous fish in the fact that their 

 embryos are entirely formed from a portion of the germ 

 which does not form part of the roof of the segmenta- 

 tion cavity, so that the cells forming the roof of the 

 segmentation cavity take wo share at any time in the forma- 

 tion of their embryos. They further agree with Osseous fish 

 (always supposing that the descriptions of Oellacher, loc. cit., 

 and Gotte, 'Archiv fiir Micr. Anat.,' Bd. ix, are correct) 

 in the floor of the segmentation cavity being formed at one 

 period by y^olk. Together with these points of similarity 

 there are some important differences. 



(1) The segmentation cavity in the Osseous fish from the 

 first arises as a cavity between the yolk and the blastoderm, 

 and its floor is never at any period covered with cells. In 

 the Dog-fish, as we have said above, both in the earlier and 

 later periods the floor is covered with cells. 



(2) The roof in the Dog-fish is invariably formed by the 

 epiblast and a row of flattened lower layer ceils. 



According to both Gotte and Oellacher the roof of the 

 segmentation cavity in Osseous fishes is in the earlier stages 



