THE FOEHATIOX OF THE LAYERS IN AHPHIOXL'S. 287 
a re-examination of the whole subject, the results of which 
are presented in this paper. He gives an elaborate account 
of the segmentation, confirming Hatschek’s account, and noting 
the presence of two circles of lai’ger blastomeres near the 
vegetative pole. When the blastula becomes flattened, he 
finds that the invagination commences asymmetrically near 
what afterwards becomes the dorsal or anterior lip of the 
blastopore. In this he entirely confirms my work, wdthout 
apparently being aware of it. But he maintains that small- 
celled ectoderm is inflected round the dorsal lip and forms 
the loof of the archenteron, thus re-aftirming Lwoff’s asser- 
tion. He arrives at this conclusion on two grounds : first, 
because of the existence of numerous mitoses near this lip (thus 
ignoring the criticisms of Samassa, of Morgan and Hazen, 
and of Klaatsch), and secondly, because of a histological 
difference between the cells forming the roof and those 
forming the floor of the archenteron. When we probe his 
figures for evidence of this difference, we find that it consists 
in the presence in the cells forming the floor of a few larger 
and more deeply staining granules of yolk than are found in 
the cells forming the roof of the archenteron ! There is, 
however, a profound difference between these latter and the 
indubitable ectoderm. Cerfontaine also re-aflirms Hatschek’s 
surmise that the blastopoi’e narrows in a seam-like fashion 
from in front backwards ; but he no more than Hatschek has 
observed such a seam, and the reasoning which leads him to 
this conclusion is certainly recondite. In the process of the 
narrowing of a wide blastopore the lateral lips must come 
together; now he shows by measurement that during this 
process the dorsal and anterior lip moves backward, in other 
words, that the embryo grows longer. Hence he concludes 
that this growth of the dorsal lip must be due to the same 
process which narrowed the transverse diameter of the blasto- 
pore ! Cerfontaine affirms in accord with Klaatsch that 
ectoderm is also invaginated at the ventral lip of the blasto- 
pore, basing his conclusion on the existence of mitoses here 
during the later stages of gastrulation. Verily Cerfontaine’s 
