GASTRULATION, FORMATION OF GERM LAYERS 77 



16. Primary yolk cells represent primary entoderm. Gastrulation 

 hiphased, the formation of the gastrula furrow being the second phase. 

 Inner layer with two components — lateral parts of mesoderm, median parts 

 of secondary entoderm. 



Hirschler, Nusbaum, Fulinski, and others are in agreement with 

 Heymons in that they consider the primary yolk cells the primary ento- 

 derm but differ as to the origin of the mid-gut epithelium which they regard 

 as a derivative of the nonmesodermal component of the lower layer. This 

 component they designate as "secondary entoderm," which corresponds 

 to the enteron (mesenteron) rudiments that Grassi first demonstrated 

 in the honeybee and that since have been described by Wheeler and 

 numerous other writers, most of whom, however, regard the yolk cells 

 as nongerm-layer derivatives. 



Hirschler and his followers in general for most pterygotes regard the 

 middle section of the lower layer as consisting of the bipolar mesenteron 

 rudiments together with a connecting median strand or of the bipolar 

 rudiments alone or of the middle strand alone. 



Ic. Primary yolk cells represent the primary entoderm. Gastrulation 

 multiphased. Definitive mid-gut epithelium, no matter in what manner 

 arising, is, in most insects, a secondary phenomenon. It may arise from 

 pure ectoderm or from the inner layer or from the ectoderm plus secondary 

 yolk cells {secondary entoderm). 



Among attempts made to reconcile the theories on germ-layer forma- 

 tion and gastrulation processes is one by Roonwal (1936, 1937) which he 

 calls the "multiphased theor^^" 



By the term "multiple-phase gastrulation" Roonwal implies a type 

 of so-called "gastrulation" of which the first phase consists in the forma- 

 tion of a primary epithelial layer (the so-called "blastoderm" in insects) 

 and of yolk cells; the latter at least in part is to be regarded as the 

 primary entoderm, in agreement with Hirschler, Schwangert, et at. The 

 second phase, lacking in most insects, leads to the formation of a first 

 primitive furrow (corresponding to a part of the gastrocoele) and of a 

 yolk-cell membrane (also a part of the primary entoderm). In the 

 third phase there is formed a further invagination furrow, and the inner 

 layer (as a part of the secondary entoderm, or entomesoderm) is differ- 

 entiated from the outer layer. The fourth phase consists finally in the 

 formation of secondary yolk cells, which are to be considered as the 

 secondary entoderm and are transitory. Phases one and four are 

 epibolic; phases two and three, invagination processes. Of this formu- 

 lation of Roonwal, Weber (1937) expressed the opinion that it is not 

 fundamentally different from the idea stated by Hirshler (1909), the 

 second phase of Roonwal being applicable only in the special case cited 

 by him. 



