114 MALCOLM LAURIE. 



It would seem to represent a modified invagination, and is com- 

 parable to the primitive streak in the chick. I was at first in- 

 clined to call this the primitive cumulus, but considering the 

 fundamental differences between Scorpions and Spiders, and also 

 the fact that, while Balfour^ places what he calls the primitive 

 cumulus at the posterior end of the embyro, Locy^ gives the 

 same name to a thickening at the anterior end, it seemed better 

 to avoid a term which might suggest erroneous homologies. 



A layer of cells (fig. 12, pr. hy.) is seen to be forming under 

 the rest of the blastoderm, though not yet extending to its 

 edges. This is well marked in the next stage, and forms the 

 greater part of the primitive hypoblast or hypomesoblast. It 

 would seem to be simply split off from the epiblast. I have 

 seen no appearance of a " down-sinking " of cells to form the 

 hypoblast, such as is described by Kowalevsky and Schulgin -^ 

 but, without the help of figures, it is not easy to be certain 

 of their exact meaning. Whether this ''down-sinking^^ is 

 supposed to take place over the whole blastoderm or only 

 at the primitive thickening is not clear from their descrip- 

 tion. 



Round the edges of the blastoderm a single layer of large 

 cells (fig. 12, s. m'.) is seen to be spreading a little way over the 

 surface of the yolk. These peripheral cells, which are at present 

 continuous with the epiblast, form later on the continuation of 

 the serous membrane. This serous membrane, or outer layer 

 of the amnion, is seen growing up as a single layer of cells 

 from the edges of the blastoderm (PI. XIV, fig. 12, s. m.). 

 It spreads over the surface of the blastoderm from all sides, 

 and its edges ultimately meet and fuse in the middle line. At 

 this stage the edges have not yet come together, and the cells 

 of the layer are still small and similar in appearance to those 

 of the rest of the blastoderm. 



The yolk is broken down to a considerable extent, and the 



1 Balfour, "Notes on the Development of the Araneina," * Quart. Journ. 

 Micr. Sci./ vol. xx, ISSO. 



2 Locy, loo. cit. 



3 Loc. cit.,p. 526. 



