38 



derm swellings may be regarded as the first rudiments of 

 the ventral chain of ganglia ; they show segmental 

 swellings, and are separated by the primitive groove. 

 The brain arises in the form of three pairs of ganglia, 

 corresponding to the eyes and the first and second 

 antennae, and belonging to three separate body-segments, 

 of which the first is, probably, the only original pre-oral 

 segment, the remaining two being apparently truuk-seg- 

 ments, originally post-oral in position, which have been 

 drawn into the head. The eye arises as a part of the 

 ectoderm, which becomes multilaminar and produces 

 from its superficial layers the cells of the cornea and 

 crystalline cones, while the lower layers give rise to the 

 retinulse and the pigment cells. 



The alimentary canal is formed by the union of three 

 separate rudiments, the fore-gut and hind-gut arising as 

 ectodermal invaginations (stomodaeum and proctodseum), 

 while the mid-gut is formed from the endoderm. The 

 fore-gut arises before the hind-gut. The mid-gut is 

 formed by the union of the two lateral bands of endo- 

 derm lying on the food-yolk, which grow round it and 

 enclose it. A pair of very large hepatic sacs are formed 

 by constriction at its sides ; they give rise later to two 

 pairs of tubes, by longitudinal constriction. From the 

 posterior section of the mid-gut arise the pair of posterior 

 dorsal diverticula. The alimentary canal is largely com- 

 posed of the fore and hind guts, so that only a small 

 portion of it is endodermal in origin. 



The heart arises in the middle of the body by the 

 fusing of two grooves which arise from a single row of 

 mesoderm cells on each side ; the ventral precedes the 

 dorsal fusion. The ostia develop at the boundaries of the 

 mesoderm segments. Eossijskaya (18) observed that the 

 genital cells were mesodermic in origin. 



