60 



commenced. It arises as a median unpaired rudiment, 

 situated between the fundaments of the hrst maxillce {ling. 

 in figs. 7 and 75). 



Mention has already been made of the origin of the 

 endoderm from certain of the cells which are left over in 

 the yolk during the formation of the blastoderm. At a 

 stage a little subsequent to the condition of the embryo 

 represented in fig. 72, these cells commence to separate. 

 They increase iu size, and, engulphing yolk, they 

 so arrange themselves as to form a definite mesenteron. 

 It is remarkable that none of the yolk is enclosed in the 

 mesenteron during the process of formation of the latter ; 

 much of it remains in the body-cavity, while the rest 

 becomes included in the reproductive organs. Whether 

 the yolk cells have anything to do with the endoderm is 

 not clear ; although they take no part in the formation of 

 the mesenteron, Claypole suggests that they may, 

 perhaps, be endoderm cells which have acquired their 

 digestive powers at a very early stage. 



The nervous system arises as a proliferation of 

 ectoderm cells in localised positions in the various 

 segments. The masses of cells so formed give rise to the 

 primitive ganglia, and they subsequently join up with 

 one another to form the brain and ventral nerve chain. 

 The brain is formed from the first three primitive ganglia, 

 which are termed respectively the proto-, deuto-, and 

 trito-cerebrum (fig. 74). There follows then in succession 

 ganglia pertaining to the segments of the mandibles, 

 maxillulse, and first and second maxillse, together with 

 three thoracic ganglia and a ganglion for each of the six 

 abdominal segments. The subsequent changes result in 

 the union of the three cerebral ganglia to form the 

 completed brain, while the remaining cephalic ganglia 

 consolidate into a siug-le g-uioiionic mass to form the sub- 



