780 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and by Wcissmann in tho Orthoptera ; wliilc that of Cynips, one of 

 the Hymenoptera, is different from these. Here the vitellus shows in 

 tho centre numerous nuclei, which emigrate to tho surface, draw the 

 protoplasm around them, and form the first cells of tho blastoderm. 

 Among tho Diptera, Chironomus shows freo nuclei in tho centre of 

 the yolk, although a protoplasmic layer has already appeared on tho 

 surface. 



Grassi's observations agree with those of Bohretsky and Tichomiroff, 

 in that, in the blastoderm, cells with numerous nuclei arc seen — a 

 sign of endogenous division. This has led him to describe the seg- 

 mentation as follows : — " The segmented egg is only a very large cell 

 with numerous nuclei surrounded by protoplasm in which is imbedded 

 a great quantity of deutoplasm." He asks, " Why is the blastoderm 

 formed on the surface instead of elsewhere ? What is tho explanation 

 of centrolecithal ova?" He explains it from a physiological point 

 of view. The cells are, in this position, in close contact with tho 

 external medium on the one hand, serving therefore for respiration 

 and excretion ; whilst their contact with the yolk within supplies them 

 with nutriment. In a similar sort of way he would explain other 

 modes of segmentation. He mentions casually that in none of tho 

 numerous eggs which he has examined fresh and by sections has ho 

 ever seen caryolytic figures. 



b. Formation of germinal layers. — He aims at showing that the 

 mode of formation of the layers in insects takes place by a process of 

 gastrulation, directly comparable to that in Peripatus, and that it does 

 not differ fundamentally from the process in other Metazoa. 



The median groove, by means of which the ento-mesoderm is 

 formed in the silkworm, may remain as a canal for some time. In 

 the bee this groove is replaced by a pair of grooves. Contrary to 

 what occurs in ordinary gastrulation, the entoderm is derived from 

 the mesoderm ; hence, to reduce the difference, he speaks of a " meso- 

 entoderm." 



c. Analogy of amnion. — The author considers the amnion, as found 

 in existing insects, as forming an integral part of the embryo's body 

 in former times. He starts by supposing it to have been part of the 

 dorsal wall of an ancestral insect, and taking no share in protecting 

 the embryo (just as is found in the lowest Articulata). The dorsal 

 wall grew more rapidly than the ventral plate, and became folded 

 over the embryo, and served to protect it; later on in develop- 

 ment, however, it became filled out, and formed merely part of the 

 body. 



Gradually it would take a greater and greater share in protection, 

 and cease to be a part of the body. Although analogous with, yet he 

 shows that it is not homologous with the vertebrate amnion. 



d. Homology of Malpighian tubes, &c, with stigmata. — The absence 

 of stigmata on the two last segments of the body is compensated for 

 by the presence of a pair of Malpighian tubes in each of these seg- 

 ments. In all insects the mode of formation of these two structures 

 agrees. The Malpighian tubes are, indeed, nearer the middle line, 

 but if the abdomen were not folded on to the dorsal surface of the 



