THE EMBRYOLOGY OF THE HONEY BEE 241 



inspection of the plates given by various investigators has proved 

 inconclusive, although it seems probable that in most insects the 

 primary yolk cells are at least of more than average size. 



The formation of "nests" of nuclei occurs in many insects and 

 has been especially mentioned by Heider (1889) in Hydrophilus 

 and by Heymons (1895) in the Dermaptera and Orthoptera, more 

 recently by O. Dickel (1904) and Nachtsheim (1913) in the 

 honey bee and by Friederichs (1906) in chrysomelid beetles. 

 Dickel and Nachtsheim state that they found mitotic figures in 

 the yolk cells (see p. 230), but both Heymons and Friederichs 

 found multiplication of nuclei to be due principally to amitotic 

 (direct) division. 



The degeneration of the primary yolk cells at an early stage in 

 the development of the embryo seems to have been noted only by 

 Lecaillon (1897) and Friederichs (1906), in the chrysomelid 

 beetles. The latter investigator has made a special study of the 

 cell elements of the yolk and finds that (i) the primary yolk cells 

 multiply at first by mitosis, later by amitosis, (2) that many of 

 their nuclei soon degenerate, forming chromatin spherules which 

 find their way out of the cells themselves into the yolk. These he 

 terms "entodermal paracytoids" in order to distinguish them from 

 similar bodies formed by cells of the tissues of the embryo. As 

 previously stated these conditions are closely approximated in the 

 honey bee. 



The yolk itself, in at least three of the large orders of insects, 

 the Orthoptera, the Coleoptera and the Lepidoptera, becomes 

 split up into polyhedral blocks or segments soon after the forma- 

 tion of the germ band. Each of these so-called segments con- 

 tains one or more yolk nuclei. In the examples of the Diptera 

 studied this process appears to be lacking, as it is also in the 

 mason bee (Carriere and Burger 1897), as well as in the honey 

 bee. In the latter insect at least the absence of yolk segmentation 

 is probably due to the fluidity of the yolk, which would make such 

 a breaking up a physical impossibility. 



The immigration of small cells or small nuclei from the blasto- 

 derm or the embryonal tissues into the yolk to form secondary 

 yolk cells has been described by a number of investigators. Hey- 

 mons (1895) and Friederichs (1906) have reviewed this subject 



