EARLY CHAXGES IN THE 0\'UM. 233 



developed from the amceboid yelk-cells is unknown, if indeed 

 any such transformation occur. Most embryologists suppose 

 that the yelk-cells are derived from the segmentation of a 

 nucleus, the germinal vesicle, which pre-exists in the &^^, or 

 to use a more modern nomenclature, from a pronucleus derived 

 from the germinal vesicle. As the first cellular elements 

 appear to travel from the interior towards the surface of the 

 yelk, the process has been termed centrolecithal segmentation. 

 Origin of the Blastula. — Korotneff [103] says, with regard to the 

 formation of the blastoderm in Gryllotalpa : ' According to Weis- 

 mann [100], the cells of the blastoderm are formed in the 

 interior of the Q.^g, and pass outwards to the surface of the yelk, 

 but are widety separated from each other, and multiply subse- 

 quently by fission until the blastoderm is closed. According to 

 my observations this process occurs as Weismann describes it ' 



(P-571)- 



This is the generally accepted theory of the manner in which 



the blastoderm is formed by centrolecithal segmentation. Its 

 improbability lies in the supposition that the centrolecithal 

 segmentation cells are formed from the germinal vesicle, and 

 that, after passing through a stage as separate amoeboid 

 wander cells, they subsequently unite and form the blastoderm. 

 Korotneff continues : ' I have not been able to observe the 

 origin of the first cell from the germinal vesicle, but do not 

 doubt the possibility {Muglkhkcit) of such an origin. The first 

 cells are not large, are amceboid, and are four or five in number ; 

 they pass to the surface of the yelk, and then undergo con- 

 siderable enlargement.' He adds : ' Such cells on the surface 

 of the yelk resemble parasitic amoebae In the second genera- 

 tion these cells become multinucleate.' 



With regard to the formation of the blastoderm itself, 

 Korotneff says : ' It first appears on the ventral surface of the 

 ^^•g in the form of numerous white specks. When we investi- 

 gate one of these, we incline to the opinion of Bobretzky that 

 they are intermediate between the amoeboid cells and the 

 elements of the blastoderm ; they are entirely without nuclei.' 

 It is from these that Korotneff believes the blastoderm is 



16 — 2 



