26 J. riRAHAM KERR. 



to the fixing- agent not having i3enetratecl sufficiently rapidly ; 

 and it also seems by no means impossible that the lower part 

 of the egg of Gymnophiona may be only apparently uncleaved 

 for the same reason. 



Segmentation Cavity. — The segmentation cavity of 

 Lepidosiren arises in the normal fashion from intercellular 

 chinks. Amia, whose segmentation otherwise so resembles 

 that of Lepidosiren, is said to develop its segmentation 

 cavity from intra-cellular spaces (Whitinan and Eycleshymer^) . 



The mode of disappearance of the segmentation cavity, the 

 blastomeres permeating it as a sponge-work, and then later 

 rounding themselves off so as to leave the diminishing cavity 

 in the form of chinks between them, resembles closely what 

 occurs in Petromyzon as described by Nuel.^ It may 

 quite possibly occur pretty generally, as in Lepidosiren 

 this stage lasts such a short time that it might easily be 

 missed. 



The two-layered character of the roof of the cavity from 

 an early stage is noteworthy. The roof, in fact, has taken on 

 its definitive epiblastic character already in the blastula 

 stage. In Ceratodus the roof is one-layered ; and in other 

 cases where it is two or three layers thick, it is usual for a 

 one-layered condition to be passed through before it becomes 

 definite epiblast (Petromyzon, Axolotl, Gymnophiona). 



Blastoporic Lip Downgrowth. — In Amphioxus it 

 has been shown that the blastopore occupies the hind end of 

 the embryo. So it is with Lepidosiren, so that we may 

 reasonably compare embryos of the two forms at the close 

 of gastrulation. 



It is commonly said that in a heavily yolked egg the 

 macromeric part has become too bulky to allow of invagina- 

 tion. This is true only in a restricted sense, there not being- 

 room for the macromeric portion to be pushed bodily within 

 the other as in Amphioxus. In such a form as Lepido- 

 siren, however, new space is continually being provided by 



1 'J. Mor|)hol.,' vol. xii, p. 336. 



2 'Aicli. Biol.,' t. ii, p. 436. 



