THE EMBRYOLOGY OF A SCORPION. 113 



the embryo. There is no doubt that these yolk-cells are 

 derived from the blastoderm in this and the next stages, and 

 do not arise in the yolk by any process of free cell-formation. 

 Kowalevsky is also of this opinion. The yolk in the Scorpion's 

 egg shows no sign of segmenting as does that of the Spider. The 

 yolk of the Spider's egg seems^ to represent the hypoblast, and 

 takes an active part in the building up of the embryo; that of 

 the Scorpion, on the other hand, remains throughout develop- 

 ment an inert mass of food-material. This fundamental 

 difference in the segmentation makes any comparison of the 

 early stages of these two groups impossible, and would 

 seem to point to an independent origin for their abundance of 

 food-material. If the segmentation in Scorpions is a modifi- 

 cation of the centrolecithal type, as would seem probable from 

 the modes of segmentation in other groups of the Arachnida, 

 it is a very extreme one, and almost all trace of its origin has 

 been lost. 



Second Period. — Formation of the Three Layers and the Em- 

 bryonic Membranes. 

 Stage D. — It is difficult to get good sections at this stage as 

 the blastoderm is often humped up at the end of the egg and 

 compressed by the ovarian tube into which it is beginning to 

 pass. In one, and only one, series of sections I have seen what 

 appeared to be a longitudinal groove in the blastoderm. This 

 primitive groove is figured by Metschnikoff" (PI. XVII, figs. 2 

 and 3), but he may have been misled by the edges of the serous 

 membrane which is growing up and might easily give the 

 appearance of a groove in surface view. If the primitive groove 

 exists, which I am inclined to doubt, as the appearance in my 

 sections may have been due to shrinking, it is a very temporary 

 structure. Towards the posterior end of the blastoderm the 

 cells are proliferating and forming what I shall call the primi- 

 tive thickening. From this primitive thickening is formed the 

 mass of hypoblast which is found later on in the tail-segment. 



1 Locy, " Observations on the Development of Agelina nsevia," 'Bull. 

 Mus., Harvard,' vol. xii. 



