646 



THE AMERICAN NATURALIST [Vol. LI 



cortical layer. Not all of the cleavage nuclei take part in 

 blastoderm formation, many of them remaining behind in 

 the yolk to aid in breaking down this substance. 



4. The Origin of the Primordial Germ Cells 

 Blastoderm formation is interrupted at tlie posterior 

 end of certain chrysomelid beetles' eggs by the segrega- 

 tion of the primordial germ cells (Fig. 4). Those cleav- 



a 



age nuclei tliat encounter the grariules of the yiole-disc do 

 not ]>i'odiu'e blnstodcDii colls, l)iit ('f)iitinn(^ tlieir migra- 

 tion ;ni(l Mi'c lin;i!lv ciil ott tiotii llic rc^t ol llic ogg as 

 di^^tm.t M,K '11h^. n. lli. ].mii,omIii! u. n,i colls of 

 whicli tlicro ai'o sixtoon. I'.acli of those colls contains a 

 portion of tlio cot tical lavor tliat includes i)ole-disc gran- 



