l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.98 



it available as nourishment for the cells of the developing embryo. 

 Because of this function, they may be termed trophonuclei or vitello- 

 phags. It would be incorrect to call them trophocytes because, like 

 the cleavage nuclei, the cytoplasm surrounding them is not marked off 

 into cell territories. In order to emphasize this point, the term tropho- 

 nucleus is used hereafter. During the preliminary cleavage divisions 

 all of the trophonuclei divide simultaneously with the other nuclei. 

 Later, however, the division of the trophonuclei is irregular. This is 

 demonstrated by the fact that while these nuclei disperse soon after 

 division, some preparations show a few lying close together in pairs. 

 This irregular division of the trophonuclei has been described previ- 

 ously for Apis by Nelson ( 191 5) . 



In some insects, such as Leptinotarsa (Wheeler, 1889) and Clytra 

 (Lecaillon, 1898), a phenomenon known as secondary yolk cleavage 

 is said to occur. In these forms the yolk is divided into polyhedral 

 masses, each of which contains a trophonucleus. This segmentation 

 serves to divide the protoplasmic reticulum of the vitellus as well, 

 so that in these forms each yolk segment with its trophonucleus and 

 surrounding cytoplasm may properly be regarded as a yolk cell. Only 

 in such cases is the term yolk cell applicable. In agreement with the 

 majority of other insects, such segmentation of the vitellus is not 

 shown in any of the numerous sections of flea eggs made during this 

 study. Therefore, for the vitellophags of the eggs of Siphonaptera the 

 term trophonucleus rather than yolk cell is correct. 



The origin of the primary trophonuclei from cleavage nuclei which 

 have remained behind in the yolk has been demonstrated in numerous 

 insects by various authors including Bobretzky (1878) for Pieris, 

 Kowalevsky (1886) for Calliphora, Wheeler (1889) for Leptinotarsa, 

 and Nelson (1915) for Apis. On the other hand Patten (1884) for 

 Ncophylax, Will (1888) for Aphis, Wheeler (1889) for Blatta, and 

 recently Du Bois (1932) for Sciara, state that all the cleavage nuclei 

 migrate to the surface so that none of them are left to produce the 

 primary vitellophags. In the eggs of fleas a few nuclei, as in the 

 majority of insect forms studied, remain behind in the vitellus and 

 these in turn give rise to the primary trophonuclei. Tikhomirowa 

 (1892) has already made this observation for the composite species 

 Pules scrraticeps. Strindberg ( 191 7) , in his study of the development 

 of Archaeopsylla erinacei, failed to find yolk nuclei at any stage. 



Not all trophonuclei are of this primary type, however. They must 

 of necessity have some other source in those forms in which all of the 

 nuclei pass to the periplasmic layer. Patten (1884) says that in Neo- 

 phylax the yolk nuclei arise at a later stage by the immigration of 



