TRACHEATA. 



439 



FIG. 199. SECTION THROUGH THK EM- 

 BRYO OF AGELENA LABYRINTHICA. 



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 

 mesoblast. 



yk. yolk divided into large polygonal 

 cells, in several of which nuclei are shewn. 



cells thick, and the cells 

 which form it are divided into 

 tivo distinct layers a colum- 

 nar superficial layer of epiblast 

 cells, and a deeper layer of 

 mesoblast 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 struc- 

 tures visible in the previous 

 stage with those visible 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 

 the part of the blastoderm where two rows of cells become first 

 established *. 



The whole region of the blastoderm other than the ventral 

 plate is formed of a single row of flattened epiblast cells. The 

 yolk retains its original constitution. 



By this stage the epiblast and mesoblast are distinctly 

 differentiated, and the homologue of the hypoblast is to be 

 sought for in the yolk-cells. The yolk-cells are not however 

 entirely hypoblastic, since they continue for the greater part of 

 the development to give rise to fresh cells which join the meso- 

 blast. 



The Spider's blastoderm now resembles that of an Insect 

 (except for the amnion) after the establishment of the mesoblast, 

 and the mode of origin of the mesoblast in both groups is very 

 similar, in that the longitudinal ridge-like thickening of the 



1 Various views have been put forward by Claparfede and Balbiani about the 

 position and significance of the primitive cumulus. For a discussion of which vide 

 self, No. 440. 



