MUSEUM OF COMPARATIVE ZOOLOGY. 275 



von Entoconcha ist voUig hell und hat eine einfache nicht doppelte 

 scharfe Contour. In seinern Innern sind keiue Granula und nichts 

 einem Keimfleck Aehnliches zu erkennen, es ist durch und durch so zahe, 

 dass man an der Existenz einer Membran zweifeln konnte. , . . Das Keim- 

 blaschen . . . gleicht daher mehr dem, was Von Baer hi den reiferen Eiern 

 des Seeigels den Kern des Eies nennt." 



Letdig ('54, p. 28) especially makes clear this feature, when, in de- 

 scribing the segmentation of the eggs of a Rotifer (Notommata), he 

 speaks of " der homogene helle Kern des reifen Eies — das Keimblaschen 

 — u. s. w.," and (p. 102) "das Keimblaschen im reifen Ei — welches 

 iibrigens keinen Keimfleck mehr hat, auch nicht ein Blaschen, sondern 

 ein homogener zaher Korper ist." 



Metschxikoff ('66, pp. 410, 411) affirms that in Miastor, after the 

 " Keimfleck " has disappeared, the germinative vesicle divides into two 

 nuclei of equal size. For Aphis the absence of the germinative dot also 

 characterizes the nucleus before division (p. 438). In Nemertes, also, 

 a very brief notice of the mature egg is given by Metschnikolf ('69", 

 p. 50, Taf. IX. B, Fig. 1, vg.), in which he mentions the germinative 

 vesicle as being large and transparent, but of an irregular form. 



Ed. van Beneden ('70, p. 39), although he infers (p. 31) that the 

 division of the nucleolus precedes the division of the nucleus, and this 

 in turn precedes that of the " germinative cell " in the case of Distoma 

 cygnoides, admits (p. 39) that in Udonella the nucleus, which the " ger- 

 minative cell " embraces and which represents the germinative vesicle 

 [first segmentation nucleus], no longer has a clearly recognizable contour ; 

 it still remains, however, as a paler spot. In the mammalian egg, too, 

 the same peculiarity of the nuclei of cleavage spheres is to be inferred, 

 inasmuch as, according to Van Beneden (p. 179), "La vesicule germina- 

 tive se conduit dans I'oeuf absolument comme les noyaux dans les spheres 

 de segmentation " ; and (p. 174), " II est incontestable aussi qu' a certains 

 moments la vesicule germinative devient tr^s-peu distincte et qu'il sou- 

 vent impossible de la distinguer." 



The division of blastodermic cells in Tegnaria domestica has been de- 

 scribed by Balbiani ('73, pp. 51, 52, and Figs. 64-66), and the deport- 

 ment of the nucleus observed. " On voit d'abord le nucleus, de pale et 

 circulaire qu'il etait, prendre une form^ allongee et devenir plus rcfrin- 

 gent." It is only the internal substance of the nucleus which divides at 

 first ; the more elastic enveloping membrane continues to hold these 

 halves together for some time (see his Figs. 65 and 66). This connecting 

 band, which Balbiani thinks to be the nuclear membrane, is unquestion- 



