134 MALCOLM LAURIE. 



become aggregated to form ganglia^ and the clieliceral ganglia 

 become fused with the cerebrum. 



(7) The tail grows out, lying along the ventral surface 

 of the abdomen. The poison-gland in its terminal segment is 

 formed by a pair of invaginations of the epiblast. 



(8) The hypoblast consists of an irregular layer under the 

 whole embryo and a solid mass at the tail end (Stage I). As 

 the tail grows the hypoblast grows into it as a tube reaching 

 down to the last somite (Stage K). 



The hypoblast forms the gut in the abdominal portion of the 

 body, growing forward in a sling of mesoblast at first as a flat 

 layer, which soon becomes bent round into a cylinder. The 

 Malpighian tubes are formed as outgrowths from the mesenteron 

 in the first post-abdominal somite (Stage M). 



The gut does not reach forward to the stomodaeum till 

 shortly before hatching, and at this period the portion of 

 it into which the liver-follicles open is not fully formed 

 (Stage N). 



(9) The stomodaeum is formed early. It lies at first in front 

 of the chelicerse (Stage I), but soon shifts its position and 

 comes to lie behind them. It extends inwards as far as the 

 back of the brain. 



(10) The proctodaeum is formed much later than the stomo- 

 dseum. It is at first a solid plug of cells (Stage M). As it 

 increases in size it appears to replace the hypoblast in the 

 last four somites. 



(11) The mesoblast consists at first of a pair of segmented 

 bands with a separate coelomic space in each somite, and also 

 one in the cephalic segment (Stage I). The ceelomic spaces 

 soon unite, and the mesoblast bands join across the ventral 

 surface. Somewhat later they extend round — the coelomic 

 space extending with them — and unite in the middle line on 

 the dorsal surface (Stage L). From the thickened baud where 

 they have united on the dorsal surface the heart is formed. A 

 portion of the coelom in the seventh segment becomes separated 

 off to form the genital tubes (Stage M). These do not open to 

 the exterior. The outer layer of the mesoblast forms chiefly the 



