430 STJMMAEY OF CUKRENT RESEARCHES RELATING TO 



directly form the dorsal skin of the larva. Apart from these envelopes, 

 the embryo exhibits in the fifth or sixth hour of its development two 

 layers — the ectoderm and the meso-endoderm of the " ventral plate." 

 With the endoderm the internal yolk-cells have nothing to do. 



(d) The splitting of the endo-mesoderm occurs in the following way : — 

 An anterior ectodermic invagination, forming the fore-gut, presses 

 upon the anterior portion of the internal layer, pushing it into the 

 yolk in the form of a watchglass-like protrusion. This becomes sepa- 

 rated from the primitive endo-mesoderm, and forms the anterior half 

 of the endoderm. An exactly similar process occurs posteriorly. The 

 arched portions of the two elevations are turned to the respective ends 

 of the embryo, the margins towards one another. These gradually 

 grow to meet, surrounding the yolk completely. Two outgrowths 

 from the margins of the watchglass-shaped rudiments grow faster and 

 meet sooner than the rest of the endoderm. The yolk-cells still 

 persist, probably loosening the yolk. It must be further noted, in 

 this connection, that the invaginations do not occur directly at, but 

 at a short distance from the poles, so that the inpushed portions of 

 the primitive lower layer does not inclose the exactly anterior and 

 the exactly posterior portions of the yolk, but sinking in, separate 

 these last-named portions from the central mass. The extreme 

 anterior and posterior portion of the yolk is thus not inclosed by the 

 endoderm, but comes to lie between the gut and the body-wall, and is 

 finally incorporated in the mesoderm, (e) The mesoderm is formed 

 from the remaining portion of the primitive lower layer. It is divided 

 (a) into two strands of cells, which lie along the growing processes of 

 the endoderm, and form the musculature of the alimentary canal, and 

 (6) of the much larger remaining portion, forming the usual structures. 



(/) General comparison. Here, as in other cases, a kind of long 

 drawn-out gastrula is formed, in which the invaginated portion forms 

 endo- and mesoderm. As in such a case as Sagitta, a median invagi- 

 nation — ^present, however, only at each end — forms the endoderm, 

 while the lateral portions furnish the mesoderm. It might even be 

 suggested that in a gastrula so much drawn out as that of insects, the 

 median (endoderm) sac is not unnaturally absent in the middle, and is 

 present only at either end. Prof. Kowalevsky pursues the comparison 

 further, both with Sagitta and with the higher Crustacea, showing 

 how the formation of the endoderm may be indeed referred to the 

 same process, viz. to a simple gastrulation. 



Optic Ganglion of some Dipterous Larvae.* — M. H. Viallanes, in 

 his third memoir on the sensory organs of Articulates, discusses the 

 optic ganglion of some dipterous larv8B. He finds that the very com- 

 plicated visual apparatus of the adult insect is completely present 

 and functionally active in the larva; it is not, however, quite so 

 completely developed, and it is entirely hidden below the muscles 

 and integuments. It consists of three chief parts — the imaginal disc 

 of the compound eye, the nerve-trunk, and the optic ganglion. 



The disc is formed like other discs ; it has an investment which 



* Auu. Bci. Nat.— Zool., xix. (1885) Art. No. 3, 32 pp. (2 pis.). 



