116 THE DEVELOPMENT OF 



" Keimblaschen," there can be no doubt, in view of what 

 is known of the eggs of other animals, that they are de- 

 rived from it. This egg-nucleus has at first a central 

 position, and hence in its segmentation we have a parallel 

 to that already pointed out in spiders and decapods. Sev- 

 eral writers (Brandt, '69; Bobretzky, 78) have failed 

 to recognize or have denied the presence of a blastema 1 

 first pointed out by Weismann ('63) in the eggs of Chi- 

 ronomus and Musca ; but this seems a point of minor mor- 

 phological importance, and its existence is readily to be 

 explained in those forms where it occurs on the ground of 

 a precocious accumulation of protoplasm on the surface 

 of the egg where it is soon to be utilized in the formation 

 of a blastoderm. The amount of protoplasm thus early 

 segregated probably differs with the species. 



In some eggs the nuclei resulting from the earlier seg- 

 mentations are certainly surrounded with protoplasm, thus 

 presenting a close similarity with the egg of Crangon ; 

 and these migrate to the surface to form the blastoderm 

 in almost exactly the same manner as in that form or as 

 in Agelena as described by Locy and the older authors. 

 Thus Bobretzky ('78), in the lepidopterous genus Por- 

 thesia, speaks of these nuclei and the surrounding proto- 

 plasm as true cells. Graber ('78) describes in several 



1 The view of a blastema here adopted is that of Weismann ('63, p. Ill) : "eine 

 diinn Schicht einer vollkommen homogen, stark liclitbrechenden, blaulichen 

 Masse," and differs considerably from that of Patten ('84, p. 563). The blastema 

 is composed of protoplasm and contains no nuclei; when the latter enter it, it is 

 converted into a blastoderm, no matter whether the cell walls are developed or 

 whether the layer has a syncytial nature. Patten says, " it is not impossible or 

 even improbable that a ' blastema' may occur in some instances without nuclei, al- 

 though at present this has not been observed to occur." Weismann in both Chi- 

 ronomus ('63, pi. vii, fig. 1) and Musca (pi. x, fig. 52, 52a) clearly shows that in 

 these forms it does occur. Metschnikoff also shows it in Caniidomyia ('66, pi. 

 xxiv, fig. 8), Aphis (pi. xxviii, figs. 3, 4, 5) and Aspidotus (pi. xxxii, fig. 2). Wit- 

 laczil ('84, p. 607, pi. xxviii, figs, a-7) confirms the observation on Aphis, while 

 Locy ('86, pp. 67-70) clearly shows its nature in the spiders. 



