574 SUMMARY OF CURRENT RESEARCHES RELATING TO 



separated oflf, and wander into the yolk and represent the cndodcrm of 

 the true gastnila. The Aphides thus preserve a primitive character. 



(2) Apical plates and bilateral symmetnj. — The bhistoderni thickens 

 at the apical pole to form tlie ajncal plate, which mainly gives origin to 

 the brain. Bilateral symmetry is soon established, and is due to 

 differences of growth and to disjilaccment in the outer germinal layer. 

 One half of the blastoderm diminishes greatly to form a thin skiu, the 

 serosa ; the other side thickens greatly, esiiecially in the apical plate. 

 As the thickening increases, the whole portion is markedly shortened, 

 and the apical pliite displaced until it occupies the inferior pole of tho 

 egg. The result is the establishment of symmetrical halves, and this is 

 soon emphasized by tho median division of the jjlate into two apical 

 lobes. 



(3) The germinal streak and the secondary yolk. — The appearance of 

 the latter obscures the relations of the former. The secondary yolk 

 penetrates the egg from the outside, but can only do so by the apposi- 

 tion of the still open blastopore against the follicular epithelium, and by 

 its concrescence with tlie same. In abnormal cases this does not occm*, 

 and such ova are most instructive. As in other Bilateralia, the closure 

 of the blastopore seems non-concentric, an inconspicuous elevation over 

 the blastopore forms a short germinal streak. Details are given to 

 show that in extant aj)hides the germinal streak is established upon the 

 previous blastopore. Will also emphasizes that the secondary yolk 

 does not really affect the endoderm cells. 



(4) Reproductive rudiments and mesoderm. — Directly after the appear- 

 ance of the at first cylindrical germinal streak, certain indifferent cells on 

 the thickened side of the germinal cylinder towards the apical plate, in- 

 crease in size, multiply rapidly, and form the reproductive rudiments. 

 Thereafter the mesoderm is formed by a process of invagination within a 

 groove, along the median line of the thickened side of the germinal 

 cylinder. The formation of endoderm and mesoderm in Aphis are two 

 successive stages of one and the same process of gastrulation. 



(5) The embryonic membranes in these and other insects are to be 

 regarded as modifications of portions of the blastoderm, and of the 

 germinal streak, which were already present in rudiment in pre-existent 

 fonns. (fi) Segments and body-cavity. Transverse grooves are seen in 

 the mesoderm plate, which divides into two lateral strands. These 

 leave the median line free except in the region of the future mouth. 

 The cavities of the segments arise by a folding of the single sheathed 

 mesoderm in consequence of the formation of appendages. They all 

 ojDen medianly. A primary body-cavity arises as a cleft between the 

 blastoderm and the ai)posed portion of the germinal streak. A secondary 

 body-cavity appears from the above folds, but the details of this can 

 hardly be given. The whole parietal mesoderm is utilized for mus- 

 culature. The intestinal peritoneum alone remains along with the 

 endodermic fatty body to line the final body-cavity. (7) The products of 

 the layers. The endoderm constitutes the mid-gut, part remains in the 

 secondary yolk, the rest forms fatty body and blood. The mesoderm 

 forms the peritoneal sheath of the gut, the heart, and above all the 

 musculatm-e. The ectoderm forms tracheae, epithelium of mouth and 

 hind-gut, skin, sense-organs, and nervous s^tem. 



