NOTES ON THE DEVELOPMENT OF THE ARANEINA. 675 



sections, a thickening of the blastoderm in the caudal region, 

 though it is not so prominent in surface views as the procephalic 

 lobe. 



A transverse section through an embryo at this stage (PI. 31, 

 fig. 12) shews that there is a ventral plate of somewhat columnar 

 cells more than one row deep, and a dorsal portion of the blasto- 

 derm formed of a single row of flattened cells. Every section 

 at this stage shews that the inner layer of cells of the ventral 

 plate is receiving accessions of cells from the yolk, which has 

 not to any appreciable extent altered its constitution. A large 

 cell, passing from the yolk to the blastoderm, is shewn in fig. 12 

 at y. c. 



The cells of the ventral plate are now divided into tivo distinct 

 layers. The outer of these is the epiblast, the inner the mcso- 

 blast. The cells of both layers are quite continuous across the 

 median line, and exhibit no trace of a bilateral arrangement. 



This stage is an interesting one on account of the striking 

 similarity which (apart from the amnion.) exists between a sec- 

 tion through the blastoderm of a spider and that of an insect 

 immediately after the formation of the mesoblast. The reader 

 should compare Kowalevsky's (Mhn. Acad. Pttersbonrg, Vol. 

 XVI. 1871) fig. 26, PI. IX. with my fig. 12. The existence of a 

 continuous ventral plate of mesoblast has been noticed by 

 Barrois (p. 532), who states that the two mesoblastic bands 

 originate from the longitudinal division of a primitive single 

 band. 



In a slightly later stage (PL 30, fig. 3^ and 3 b} six distinct 

 segments are interpolated between the procephalic and the 

 caudal lobes. The two foremost, c/i and pd (especially the first), 

 of these are far less distinct than the remainder, and the first 

 segment is very indistinctly separated from the procephalic lobe. 

 From the indistinctness of the first two somites, I conclude that 

 they are later formations than the four succeeding ones. -The 

 caudal and procephalic lobes are very similar in appearance, but 

 the procephalic lobe is slightly the wider of the two. There is 

 a slight protuberance on the caudal lobe, which is possibly the 

 remnant of the cumulus. The superficial appearance of seg- 

 mentation is produced by a series of transverse valleys, sepa- 

 rating raised intermediate portions which form the segments. 



