EMBKYOLOGY OF CLIiPSINE. 241 



process, as represented by Oellacher was therefore only a pre- 

 cucious division of nucleij to be followed sooner or later in each 

 case by a corresponding cleavage of the gerni-substance, analo- 

 gous to what happens in the eggs of insects. 



While this view is entirely consistent with the uninuclear 

 character of the cell, it cannot be accepted as Biitschli has 

 already shown (rVir)- '^^^ peculiar clusters of nuclei found by 

 Balfour ( j^'uV) in the floor of the cleavage-cavity (Elasniobranch 

 Fishes) are not to be confounded with the small bodies under 

 consideration. According to Balfour these nuclei possess distinct 

 nucleoli, and become the nuclei of blastoderm cells. The idea 

 expressed by Biitschli (ytjV) ^^at these clusters are nuclei iu 

 process of coalescing, has not the slightest shade of probability 

 in its favour. 



Van Beneden (tVV) regards the Spermakern of Hertwig as a 

 Tiucleolus, and the clear spot in which it arises as a nucleus. 

 He maintains the same in respect to the two pronuclei seen in 

 the egg of the rabbit, iu both of which one or more nucleoli 

 arise. Van Beneden calls attention also to the characteristic 

 lead-grey or blackish colour, given to the pronucleus by osmic 

 acid — precisely what 1 have above mentioned as an evidence of 

 the nuclear nature of these bodies. 



Auerbach (yut)> to whom we are indebted for the first accu- 

 rate knowledge of the origin of the cleavage nucleus, recognized 

 the two " clear spots " which he saw arise in the two poles of 

 the egg (Ascaris) as nuclei, and the two or three corpuscles that 

 arose in the centre of each as nucleoli (2-5-4) Besides, he repre- 

 sented these nucleoli as persisting until the transformation of 

 the nucleus into the first cleavage spindle. 



In the same way Strasburger ( 24-, Pi. XXVIII, figs. 69 — 71) 

 represents the cleavage nucleus as arising by the union of two 

 pronuclei, in each of which small nucleoli are figured. 



In both these cases the nucleoplasm persists, as in the case of 

 Clepsine. Hertwig's investigations upon Toxopneustes (21, 

 Taf XII, figs. 15 20) prove conclusively that here also the 

 nucleoplasm of the " central area " persists and stretches to form 

 the bistellate figure before the so-called nucleus disappears. 



The cases here referred to will suffice to show that evidence is 

 not wanting in favour of the interpretation of the three cor- 

 puscles as nucleoli. 



/. Primary Cleavage-amphiaster.—Eig. 72,P1. XIV, represents 

 a stage of the same date as fig. 71 (2 h. 30 miu.), but it is 

 plainly more advanced. The nucleus has passed from the 

 spindle -form of stage 71 to a biscuit form. The nucleoli are 

 no longer visible, but stretching through the centre of the 

 biscuit-shaped figure, which is somewhat inclined to the main 



