EARLY DEVELOPMENT 35 



regarded by some writers as the primary entoderm. This matter is 

 discussed in the chapter on gastrulation. Certain cells, termed "para- 

 cytes" by Heymons (1895a), liberated into the yolk from the germ band 

 are not likely to be confused with secondary yolk cells. They are derived 

 from both ectoderm and mesoderm and disintegrate during embryonic 

 life. In sections of an early embryo they have the appearance of super- 

 numerary cells that are crowded out of the tissues. 



SECONDARY YOLK CLEAVAGE 



During gastrulation or later the yolk undergoes a cleavage into large 

 polyhedral or spherical masses, each with one or more yolk nuclei within. 

 This type of cleavage begins anterior to or in the immediate vicinity of 

 the embryo and also under the serosa, but later it extends to the rest of 

 the yolk. Each spherule is surrounded by a delicate j^olk membrane. 

 Yolk cleavage of this type occurs in the Orthoptera, Dermaptera, Coleop- 

 tera, Lepidoptera, and Mallophaga. Kessel (1939) would limit the use 

 of the term "yolk cell" to what is here designated as "yolk spherule," 

 but the term "yolk cell" is so firmly fixed in existing literature as a 

 synonym of " vitellophag " that it will be difficult to eliminate. 



GERM CELLS 



The early history of the germ cells in the Eutracheata has been 

 studied in a number of orders, especially the Diptera, Coleoptera, and 

 Hymenoptera. Robin (1862) and Weismann (1863) described them 

 under the name of "polar globules" or "pole cells." A few j^ears later 

 Leuckart (1865) and Metschnikoff (1866) identified the pole cells in the 

 cecidomyid Miastor metraloas, and Ritter (1890) those in Chironomus, as 

 the true primordial germ cells (Fig. 313). Since then they have been 

 found in numerous species of insects. In certain chrysomelid beetles 

 and nemocerous flies, by reason of the presence of the oosome (germ-track 

 plasma), the primordial germ cells are easily distinguishable from the 

 somatic cells and therefore can be traced from the time of their appear- 

 ance until they become mature eggs and sperms. To some writers this is 

 considered a clear demonstration of the early differentiation of the germ 

 cells set apart for the preservation of the race as distinguished from the 

 somatic cells set apart from the maintenance of the individual, as postu- 

 lated by Weismann in his theory of the continuity of the germ plasm. 

 Some writers believe that the areas of the periplasm into which a cleavage 

 nucleus enters determine what that particular nucleus will later produce. 

 As Hegner (1917) expresses it: 



The kind of tissue which develops from any part of the egg depends upon 

 the kind of cytoplasm encountered by the cleavage nuclei. The distribution of 

 the nuclei is entirely adventitious. They are potentially alike, that is, toti- 



