TR ACHE AT A. 



363 



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 ap- 

 pearance. 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 fisr. 200 A. Though my observations on this stage are not quite 



11111 "I " 



satisfactory, yet it appears to me probable that there is a longi- 

 tudinal thickened ridge of the blastoderm extending from the primi- 

 tive 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 blastoderm; 

 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 cau- 

 dal lobe behind, and an inter- 

 mediate region marked by about 

 three transverse grooves, indi- 

 cating a division into segments. 

 This plate is throughout two or 

 more rows of cells thick, and the 

 cells which form it are divided 

 into two distinct layers a co- 

 lumnar superficial layer of epiblast 

 cells, and a deeper layer of rneso- 

 blast cells (fig. 203 A). In the 

 latter layer there are several very 

 large cells which are in the act 

 of passing from the yolk into the 

 blastoderm. The identification 

 of the structures visible in the 

 previous stage with those visible rnesoblast. 



Fio. 199. SECTION THROUGH THE EM- 

 BKYO OF AGELENA LABYEINTHICA. 



The section is from an embryo of the 

 same age as fig. 200 A, and is represented 

 with the ventral plate upwards. In the 

 ventral plate is seen a keel-like thickening, 

 which gives rise to the main mass of the 



yk. yolk divided into large polygonal 

 cells, in several of which nuclei are shewn. 



in the present stage is to a great 



extent a matter of guess-work, 



but it appears to me probable 



that the primitive cumulus is still present as a slight prominence 



visible in surface views on the caudal lobe, and that the other 



thickened patch persists as the procephalic lobe. However this may 



be, the significance of the primitive cumulus appears to be that it is 



