ZOOLOGY AND BOTANY, MICKOSCOPYj ETC. 573 



developed extend, later on, througliont tlie whole length of the egg. 

 They all grow dorsally and ventrally, and so come to completely inclose 

 the yolk and form the epithelium of the mid-gnt. The coelom is formed 

 by the separation of these pads from the mesoderm. 



The trachere arise as segmental invaginations, which extend back- 

 wards and forwards, and unite into one longitudinal trunk ; the segmental 

 invagination-orifices close up. The nervous system arises in three 

 parts, a median invagination of the ventral surface of the ectoderm, and 

 two lateral thickenings. 



Early Stages in Development of Egg of Fly.* — Dr. H. Henking 

 has investigated the early stages in the development of the fly's egg, 

 with especial reference to free nuclear formation. In the prepared 

 unripe egg the germinal vesicle may be seen as a colourless sphere 

 floating in the egg-contents, which are distinctly coloured by carmine ; 

 it has a sharp, simply contoured wall, and contains very fine granules 

 and some clear vesicles, as well as an excentric and distinctly coloured 

 germinal spot, which is provided with vacuoles. The large nuclei of 

 the nutrient cells are very striking, and are very rich in chromatin • 

 these cells and nuclei have almost altogether disappeared from ripe 

 eggs ; their chromatin has probably been taken up by the egg-cell. In 

 the ripe egg there is but a rudiment of the germinal vesicle in the shape 

 of a small coloured corpuscle surrounded by a clear space. Only a few 

 observations were made on the polar globules. In most cases of fertili- 

 zation it would seem that four spermatozoa enter the egg. Nothing 

 definite can be said as to the fate of the female chromatin substance. 



The first yolk-cells are formed in two clouds of protoplasm by free 

 cell-formation. The first two cleavage-nuclei appear as clear bodies 

 with an equatorial zone of distinct chromatin filaments ; the succeeding 

 divisions follow very rapidly, owing to the number and rapidity of the 

 divisions of the embryonic cells. 



By free nuclear formation, the author means all those cases of the 

 formation of nuclei, in which the substance of the mother nucleus does 

 not pass directly, and unaltered, into the daughter-nuclei. The drop- 

 like bodies which are seen in the developing egg must not be called by 

 the same name as the nuclei which contain chromatin, for they have not 

 the same chemical composition. The former have no membrane. The 

 supernumerary spermatozoa break up, and the first primitive nuclei 

 arise in their place. The disappearance of the marginal portions of 

 chromatin, and the formation of a colourless spot is explained by the 

 chromatin having entered into another chemical combination. When 

 there have been chromatin particles formed from the spermatozoa, 

 cleavage -spindle, and yolk, there may arise free nuclei which take part 

 in the conversion of yolk nuclein into nuclear nuclein. 



Development of Aphides.f — Herr L. Will reports the results of his 

 recent investigation of the important but difficult subject of the develop- 

 ment of the viviparous aphides. 



(1) Gastrulation. — The blastoderm, as the author and Metsehnikoff 

 have previously noted, does not overgrow the whole of the surface, but 

 leaves a roundish spot at the lower pole. At the margin of this lower 

 aperture, an active proliferation occurs; the new-formed cells are 



* Zeitschr. f. Wiss. Zool., xlvi. (1888) pp. 289-336 (4 pis.), 

 t Biol. Centralbl., viii. (1888) pp. 148-55. 



