56 SUMMARY OP CURRENT RESEARCHES RELATING TO 



the probable origin of tbeir nuclei from part of the lower half of the 

 germinal vesicle. He distinguishes these polar globules, of course, 

 from vesicules directrices or " Sichhmg sharper, " not yet certainly 

 seen in insects, and notes the importance of not confounding them 

 with small drops of protoplasm of varying number, which appear 

 without nucleus, division, or morphological constancy, at the anterior as 

 well as at the posterior pole, and which are probably in part squeezed 

 out by the contraction of the vitellus. 



Soon after the formation of the polar globules, the blastoderm- 

 cells appear, preceded by wavy protrusions of the clear outer plasma. 

 Balbiani is unable to decide as to the origin of the blastoderm nuclei, 

 though inclining to believe that they result from the division of the 

 germinal vesicle, move outwards to the periphery, there gather proto- 

 plasm round them and form cells. 



As the blastoderm becomes more distinct the polar globules begin 

 to sink into the vitellus, but the exact way in which this is ac- 

 complished Balbiani is unable to determine. As the insinking con- 

 tinues, the blastoderm cells become further differentiated at the 

 expense of an internal plasmic layer (hlasteme germinatif interne, or 

 couche plasmique secondaire), which appears between the young blasto- 

 derm and the granular central vitellus. The mode of this further 

 growth is fully described. 



He follows in detail the invagination of the polar globules and of 

 the two folds of the blastoderm, the ventral — thickening to form the 

 caudal portion of the embryo, the dorsal — thinning to become the 

 delicate caudal fold of the amnion ; and further, the various stages 

 by which the reproductive cells, as the polar globules prove themselves 

 to be, find their final position in the hatched larva. After invagina- 

 tion the eight cells are reduced, probably by fusion, to four, bilaterally 

 arranged in pairs. Each of the four naked cells contains four or 

 more nuclei, while the protoplasm shows as yet at most only hints of 

 division. In a larva five days old a delicate membrane round the rudi- 

 mentary sex-gland can be detected, and this is prolonged at each end 

 to form anteriorly the dorsal attaching filament, and posteriorly, 

 probably the rudimentary excretory duct. 



In some larvae the gland thus formed is narrow and fusiform, in 

 others, bluntly pointed and oval. This slight difference is the first 

 hint of sexual differentiation. Each gland exhibits a transverse 

 partition, and nuclei surrounded by zones of protoplasm, but these 

 daughter-nuclei and cells in what turns out to be the male gland are 

 smaller and more numerous than in the rudimentary ovary. At a 

 later stage Balbiani observes in both glands, not simple cells, but 

 groups of pear-shaped cells, radiately arranged round a central mass 

 from which they have probably been budded off. This arrangement 

 in the female he compares to the well-known disposition of elements 

 in the terminal ovarian chamber of the adult insect ; each radiately 

 arranged group of cells would be homologous with the contents of a 

 terminal chamber. In the similar rosettes in the male, the radiately 

 arranged cells are, like their predecessors, smaller and more numerous 

 than those of the young ovary. He compares the male rosettes and 



