Z W. F. E. WELDON. 



blastic type, has already been fully described and figured by 

 Kupffer and Benecke^ and by Balfour. ^ Neither of these ob- 

 servers describes a segmentation cavity; but Hoffmann^ states 

 that during the latter stages of segmentation a cavity is present, 

 the floor of which is formed by the yolk, the roof by the lower 

 layer cells. Towards the close of segmentation it disappears. 



This cavity Hoffmann considers equivalent to the segmenta- 

 tion cavity of the Icthyopsida. 



I have observed cavities similar to that described by Hoff- 

 mann, but I have been unable to satisfy myself that they were 

 not due to the action of the hardening reagents employed. 

 The cavity described by Professor Hoffmann differs strikingly, 

 as he himself points out, from the segmentation cavity of other 

 vertebrates, in the fact that its floor is never formed of lower 

 layer cells. 



At the close of segmentation the blastoderm consists of a 

 superficial layer of epiblast cells, which is generally stated to 

 be a single cell thick ; in my sections, however, the arrange- 

 ment is very irregular, the epiblast being in some places two 

 cells deep, in others more. 



Beneath the epiblast is an irregular sheet of lower layer cells ; 

 this layer is in many places two or three cells deep, and the 

 cells of which it is composed are large, irregular, loaded with 

 yolk-granules, many having two or even more deeply-staining 

 nuclei. 



In the centre of the blastoderm the epiblast cells become 

 more columnar than in the peripheral parts, and the lower 

 layer cells become slightly more regular in their arrangement. 

 An oval area pellucida is thus formed. 



Hoffmann finds at this stage a marked thickening of the 

 lower layer cells at the posterior extremity of the blastoderm. 



The posterior region of the area pellucida now becomes dis- 



' Kupffer u. Benecke, 'Die erste Entwicklung am Ei der Reptilien.' 

 Konigsberg, 1878. 



' Balfour, "On the Early Development of the Lacertilia," 'Quart. Journ. 

 Mic. Sci.,' vol. xix. 



3 Loc. cit. 



