438 ARANEINA. 



to be situated not at the centre, but on one side of its yolk 

 segment. 



The further description of the development of Spiders applies 

 more especially to Agelena labyrinthica, the species which 

 formed the subject of my own investigations. 



The first differentiation of the blastoderm consists in the 

 cells of nearly the whole of one hemisphere becoming somewhat 

 more columnar than those of the other hemisphere, and in the 

 cells of a small area near one end of the thickened hemisphere 

 becoming distinctly more columnar than elsewhere, and two 

 layers thick. This area forms a protuberance on the surface of 

 the ovum, originally discovered by Claparede, and called by him 

 the primitive cumulus. In the next stage the cells of the 

 thickened hemisphere of the blastoderm become still more 

 columnar; and a second area, at first connected by a whitish 

 streak with the cumulus, makes its appearance. In the second 

 area the blastoderm is also more than one cell deep (fig. 199). 

 It will be noticed that the blastoderm, though more than one 

 cell thick over a large part of the ventral surface, is not divided 

 into distinct layers. The second area appears as a white patch 

 and soon becomes more distinct, while the streak continued to 

 it from the cumulus is no longer visible. It is shewn in surface 

 view in fig. 200 A. Though my observations on this stage are 

 not quite satisfactory, yet it appears to me probable that there 

 is a longitudinal thickened ridge of the blastoderm extending 

 from the primitive cumulus to the large white area. The section 

 represented in fig. 199, which I believe to be oblique, passes 

 through this ridge at its most projecting part. 



The nuclei of the yolk cells during the above stages multiply 

 rapidly, and cells are formed in the yolk which join the blasto- 

 derm ; there can however be no doubt that the main increase in 

 the cells of the blastoderm has been due to the division of the 

 original blastoderm cells. 



In the next stage I have been able to observe there is, in the 

 place of the previous thickened half of the blastoderm, a well 

 developed ventral plate with a procephalic lobe in front, a 

 caudal lobe behind, and an intermediate region marked by 

 about three transverse grooves, indicating a division into 

 segments. This plate is throughout two or more rows of 



