Second Rotation 169 



the tliird is the premanclibular segment, whose ap- 

 pendage, undeveloped in all insects of post-embr3^onic 

 age, would apparently correspond to the second pair 

 of antennae of Crustacea. The development of this 

 region is peculiar, in that it proceeds from a central and 

 two lateral masses. The central tract is believed to be 

 relatively primitive ; it becomes divisible into three, or, 

 according to some authors, four successive lobes, each 

 with its "own ganglionic mass. The lateral tracts, which 

 are outgrowths from the central one, yield the compound 

 eyes, the antennae, and the ganglia specially associated 

 with these organs. The antennae are at first placed on 

 each side of the mouth, or even behind it ; they grow 

 forwards and soon become pre-oral. It has been thought 

 that there was primitively a pair of simple eyes to each 

 of the three median segments ^ Behind these come the 

 three or four jaw-segments, whose ganglia fuse to form 

 the suboesophageal. Only the appendages of the jaw- 

 segments are usually well developed ; of the segments 

 themselves doubtful remnants have been traced in the 

 occipital or gular regions of the head. The brain- 

 segments are therefore excessively developed clorsally 

 and laterally, but incompletely ventrally ; while the 

 jaw-segments are incomplete dorsally, and only dis- 

 tinguishable by their ventral appendages. The three 

 thoracic segments normally bear legs, and each encloses 

 its own ganglion. The abdominal segments often bear 

 appendages in some stage or other, but the morphological 

 value of these appendages is not yet established. (See 



P- 33-) 



To>vards the middle of the second day the embryo, Second 

 which has for some hours been so placed in the egg that 

 the head- and tail-ends of the ventral plate lay on the 

 convex side, slowly rotates a second time through 180". 



In the course of the second day the fore- and hind-gut Fore- and 



, . , hind-gut. 



form. An invagination appears at the tail-end 111 tlie 

 inturned extremity of the ventral plate. The proximal 

 wall of the invagination is thick ; the distal wall (nearer 

 to the end of the body) is continuous with the extra- 

 embryonic blastoderm. The fore-gut forms in the same 



' Patten, 'The Eyes of Acilius,' Journal of Mon^holocjij, vol. ii, 1888. 



