MUSEUM OF COMPARATIVE ZOOLOGY. 403 



As Oellacher made considerable advance on his predecessors, owing 

 largely to the use of sections, so Flemming certainly followed rational 

 methods in watching closely the results of staining, and by this means 

 he came near anticipating the later discoveries of Hertwig. As the re- 

 sult of his own observations and those of Oellacher, Flemming insists 

 upon a fundamental importance for the polar globules, principally on 

 account of their constancy and their reaction with staining fluids. Of 

 the manner in which they arise, he still leaves us in doubt. Flemming's 

 principal objection to considering them the eliminated germinative vesi- 

 cle had been the discovery, at a little later stage, when the radial figures 

 preceding the first cleavage have already appeared {loc. cit., Taf. III. 

 Fig. 2), of a small, deeply staining body (nuclear disk of first segmenta- 

 tion sphere) in the middle of the yolk. Since Fol has deduced a similar 

 nuclear remnant directly from the old nucleus of the egg (germinative 

 vesicle), it is hard to understand, he says, how almost the whole of it 

 (germinative vesicle) should have been eliminated. Auerbach's discov- 

 eries, however, now come to the rescue, and this " remnant of a nucleus " 

 (Fol) may be supposed to descend from the secondary nucleus (nucleus 

 of the first cleavage sphere), thus leaving no objection to a total elimina- 

 tion of the germinative vesicle. Influenced by the prevailing dogma 

 that the formation of the polar globules is essentially a process of elimi- 

 nation, perhaps a necessary elimination, Flemming naturally raises the 

 question if such does not precede each act of proliferation, — especially 

 since the morphological extinction of the nucleus precedes every division 

 of a segmentation cell, — and, in the absence of any evidence of the elimi- 

 nation of polar globules or their like during segmentation, answers it 

 only by declaring that " nothing compels the assumption that every pro- 

 cess which characterizes the beginning of the construction of a body 

 Kiust accompany each individual phase of its advance." 



The preliminary account given by Butschli ('75, pp. 203, 208-210) 

 from studies on several nematodes and two pulmonate mollusks — Lym- 

 nseus and Succinea — embraces a valuable contribution toward deter- 

 mining the fate of the germinative vesicle and the origin of. the polar 

 globules. Soon after the eggs of non-parasitic nematodes enter the 

 uterus, the germinative vesicle, having lost its dot, becomes less distinct, 

 and approaches the surface of the yolk, usually at the equator, but some- 

 times nearer the vaginal pole. The surface of the yolk here becomes 

 depressed to receive the clear mass of the vesicle, which lies, as it were, 

 sunk in a pit of the granular yolk. In the case of Tylenchus, as the 

 vesicle reaches the surface a %m^\\, round, rather dark body is pushed 



