418 A. A. W. HUBRECHT. 
by the Utrecht Society for Arts and Sciences (30), and accom- 
panied by six plates, in which the most important of the 
numerous sections through different stages have been figured. 
At the request of the Editor of this Journal I now give a full 
account of the contents of this more extensive paper, and a re- 
production of the last of the six plates, in which the principal 
results are combined into fifteen diagrammatic tracings. 
a. The earliest Developmental Stages and the 
Derivates of the Primary Epiblast. 
Up to the stage when the invaginate gastrula has appeared 
(Pl. XXII, fig. 1) no difference obtains between Barrois’s 
description (21) and my own. The hypoblast cells are larger 
sized than the epiblast cells, and at an early period, when the 
blastopore is still wide and spacious, the first traces of differen- 
tiation in the epiblast appear, which have escaped Barrois’s 
notice, although the subsequent stages were again very cor- 
rectly interpreted by him. I here refer to the formation of 
the discs of secondary epiblast. The first indication of the 
formation of these discs can be clearly traced in sections.! 
At four different spots, of which two can only be seen at a 
time in transverse or longitudinal sections, we notice that the 
cubic epiblast cells divide lengthways, thus becoming palisade 
cells (Pl. XXII, fig. 2). No transverse division (delamination) 
is here noticed, such as we will have to describe by-and-by in 
other parts of the epiblast. When these four areas have de- 
finitely obtained this changed aspect the surrounding epiblast 
cells commence to overcap them (Pl. XXII, fig. 3), and thus 
they are very soon completely enclosed inside the primary 
epiblast. We have then the four discs so well known in the case 
of the Desor larva, and often compared to the four invaginate 
portions of the primary epiblast of Pilidium. I must, however, 
emphatically remark that the ultimate body wall of the young 
1 For the methods which I have followed in order to obtain these sections, 
and direct them according to a given plane, I must refer the reader to the 
Dutch treatise cited above (80). 
