410 NOTES AND MEMORANDA. 



sky's hypothesis, from the ogg-nucleus, but it seemed probable that 

 they were formed freely in the interior of the egg after fecundation ; 

 in sections of the new-laid egg, small lumps of protoplasm were 

 found, the appearance of which convinced the author that in them the 

 future amo3boid germ-cells were to be sought. 



2. Tichomirotf differs from former workers at the embryology of 

 insects, in denying the formation of the muscular layer by an invagina- 

 tion of the ectoderm. He states that a temporary sinking in of the 

 outer layer takes place in the position of the future dorsal groove, but 

 that he has never observed a true invagination. The cells of the 

 muscular layer are formed by division from those of the ectoderm, the 

 process taking place at any point in the germ-lamella, and not only in 

 the middle. 



The yolk-spheres, with their numerous nuclei, which are seen in 

 all stages of development up to hatching, are true formative cells, at 

 the cost of which the mesoderm grows. They also give rise to 

 migratory cells, the latter being not unfrequently found in the 

 interior of the yolk-cells. 



4. To the endoderm, i. e. the epithelium of the midgut, the author 

 assigns a very remarkable origin. When the mesoderm has under- 

 gone segmentation, it undergoes complete solution of continuity along 

 the middle line of the germinal streak, so that two distinct meso- 

 dermal plates are formed, as in Worms and in some Vertebrates. These 

 mesoderm plates then begin to grow towards the dorsal side of the 

 embryo, forming a pair of lamellte, which soon separate from the rest 

 of the mesoderm as midgut-j^lates (Mitteldarmlamellen), and then 

 grow ventralwards as well as dorsalwards. At the same time their 

 most superficial layer of cells become so differentiated as to form the 

 flattened epithelium of the future midgut: the remaining cells become 

 the thin muscular layer. The two midgut-lamellfe then begin to 

 approach each other both dorsally and ventrally : ventrally they soon 

 unite, and so close in the gut below ; dorsally, on the other hand, they 

 remain separate for a long time, and undergo an extraordinary change 

 in their mode of growth. Their outer or muscular layer, in fact, 

 begins to grow faster than the inner or epithelial layer, and soon 

 extends beyond the latter, so that now each midgut-lamella consists 

 of two parts— a ventral two-layered plate, united with its fellow below, 

 and a dorsal purely muscular band. The two muscular bands thus 

 differentiated from the midgut lamellre grow upwards, diverging some- 

 what from one another, until they are in close contact with the dorsal 

 wall of the embryo, when they curve inwards towards one another and 

 unite completely. In this manner a double tube is produced, having, 

 iu cross section, the form of the figure 8. Of the two tubes, which 

 are at first in free communication with one another, the ventral one, 

 composed of an outer muscular, and an inner epithelial layer, becomes 

 the midgut: the dorsal tube, wholly muscular, becomes the dorsal 

 vessel of the insect's blood-system. 



5. The silk glands take their origin simultaneously with the 

 trachere, and in their earlier stages resemble the latter completely, 



6. There are no cephalic trachete, the invaginations of the ecto- 



