Formation of Blastoderm 



159 



usual way (fig. 117). Segmentation begins at the hinder 

 pole about two hours after egg-laying. A few large 

 nuclei appear in successive pairs at the surface of the 

 egg, the protoplasm gathers round each, and forms con- 

 stricted masses, which afterwards bud off, become free, 

 and divide into four and eight (figs. 118, 119). After this 

 the contained nuclei divide without separating, so that 

 when we consider the subsequent 

 ^ history of the buds, we shall find 



that they consist of eight large cells, 

 with four nuclei apiece. These cells 

 have often been colledi polar cells ov 

 polar globules. In view of their 

 ultimate destination, and to prevent 

 confusion with the bodies broken up 

 within the egg during maturation, 

 we may call them the sexual germs'^. 

 The differentiation of sexual germs 

 in this very early stage, before a 

 blastoderm has been formed, is a 

 phenomenon as yet observed only 

 in Diptera In Distomum the germ- 

 cells (ova ?), which give rise to 

 Fig. 118.— I, First forma- ^^xQ sTDorocvsts and rcdiac, are set 



tion of sexual germs [sg) l j ^ 



and somatic nuclei (nc). anart verv carlv, at the end of 



2, Later stage, both the '^ ." 



sexual germs and the so- cleavage (cf. Sagitta). 



matic nuclei having in- . t ,i i _, . 



creased by division. (From At the time wlieil the SCXUal Formation 



Hitter, 1890, «gs. 5, 6.) , ^ n ^ l^ 11 ^ of blasto- 



germs become defined, the yolk con- derm, 

 tains several scattered nuclei, which can sometimes be 



' Tlie sexual germs in the egg of Chironomus were first observed by 

 Robin (i862\ Weismnnn (1863) observed that they subsequently became 

 withdrawn into the yolk. Metschnikoff vi866) believed that in Cecido- 

 myia he had observed the derivation of the reproductive organs from 

 these germs. Balbiani (1885) traced the development of the germs into 

 the testes and ovaries of Chironomus. Eitter (1890) demonstrated the 

 validity of Balbiani's conclusions by means of thin sections. 



