ON THE DEVELOPMENT OP ARANEINA. 65 



establishment of the ventral plate during this period. The ectoderm 

 of the cephalic lobe is very mach thickened as shewn in figs. 22 and 

 23. This process of thickening proceeds backwards as two longitudinal 

 bands, one on each side of the body, along the inner side of the attach- 

 ment of the appendages in the thoracic and abdominal segments, finally 

 meetino" each other in the caudal lobe. These two l^ands are the first 

 rudiments of the ventral nerve chain. Thus it is continuous from 

 the first with the cephalic thickening above mentioned which becomes 

 the brain, as in the case of scorpions observed by Kowalevsky and 

 Schulofin.* This is not in accordance with the view of some authors 



a 



who maintain that the brain and the ventral nerve cords are formed 

 independently of each other. The cells composing the ventral cords 

 aggregate in each segment and give rise to the ganglia. 



The cephalic thickening of the ectoderm is now divided into two 

 semicircular lobes (PL XIII, figs. 20, 21). Near the front edge of 

 these lobes, there is formed on each side a semicircular groove (sem. 

 gr.). This paired groove which is cut ofi' from tlie ectoderm is the 

 chief origin of the brain. I'riice** compares it with the amniotic 

 f )ld of insects ; but the comparison is certainly not justifiable. 

 Kowalevsky and »Schulgin found that in Scorpions the ectodermic 

 invagination comparable to the amniotic fold of insects is distinct from 

 and formed earlier than the semicircular groove, which is no doubt 

 homologous with the similar groove of the spider, as it also gives 

 rise to the brain. Sections of the semicircular groove are represented 

 in fig. 23, PI. XIY. 



Besides the semicircular grooves, there is a pair of small ecto- 

 dermic invaginations in the posterior part of the head near the 

 outer bor.ler (PI. XIII, fig. 20, PL XIV, fig. 23, lat. v). So far as I 



* Kow.ilevsky and Sohulgin — Zur Entwicklungsgeschichte des Scorpions, Biolog, Cen- 

 tralbl. VI. 



** Bruce — On Insects and Arachnids. 



