110 M. Oscar von Grimm on the Agamic Reproduction 



vesicle into germ-nuclei, and the envelopment of these bj the 

 formative vitellns, and the formation of the blastodermic cells, 

 which certainly appear to be the analogues of the spherules of 

 segmentation ? Is it, then, possible to overlook the identity 

 between the process of segmentation and the formation of the 

 blastoderm ? It is true that in the insect-ovum numerous 

 cells (representing spherules of segmentation) are formed at 

 once, after the germinal vesicle, which lies in the nutritive 

 vitellus, has divided into numerous germ-nuclei ; but this is 

 caused by the circumstance that the insect-ovum is neither 

 holoblastic nor meroblastic; and the later separation of the 

 formative vitellus is also a consequence of this anomaly in the 

 development of the spherules of segmentation as we may 

 call it. 



Clapar^de*, Leuckartf, and Metschnikowf have, indeed, 

 already expressed the opinion, in opposition to Weismann, 

 that the formation of the blastoderm is a process analogous to 

 segmentation ; but to me these two processes appear to be 

 perfectly identical, and I have therefore ventured to dwell at 

 some length upon this question. 



The inner blastodermic layer, which has become converted, 

 in consequence of the transverse division of its cells, into a 

 finely cellular, many-layered cell-mass, now thickens, but 

 chiefly in the region of the convex side of the ovum. 



In consequence of the formation and thickening of the 

 blastoderm, the size of the total contents of the ovum increases, 

 and they now occupy the polar spaces. The four polar cells 

 consequently become immersed in the blastodermic layer, from 

 which, however, they shine forth very distinctly. Their meta- 

 morphosis into the ovaries we have already witnessed, and 

 therefore we refer to them no fm'ther. 



In consequence of the continuous division of the cells the 

 blastoderm has become converted into a finely cellular, 

 strongly refractive mass surrounding the nutritive vitellus on 

 all sides. Immediately after this we observe the formation of 

 the germinal streaky which is formed by a thickening of the 

 inner blastoderm in the ventral surface of the ovum. The 

 development of the germinal streak consists first of all in the 

 formation of the so-called caudal pad. At the same time the 

 primitive caudal furrow appears ; but this soon disappears, 

 having no further consequences. The form of the caudal pad 

 may be best recognized from the form of the nutritive vitellus. 



* Recherches sur revolution des Araign^es. 



t "Die Fortpflanzimg und Entwicklung der Pupiparen," Abhandl. 

 naturf. Gesellscli. zu Halle, iv. p. 210. 



X Embryologische Studien an Insecten, pp. 93-95. 



