GASTRULATION, FORMATION OF GERM LAYERS 75 



The following tabulation sets forth the chief points involved in some 

 of the prevailing theories. 



1. Primary yolk cells represent the primary entoderm. The entoderm being 



accounted for, the so-called "blastula" is really the gastrula. 



a. Gastrulation completed when primary epithelium (so-called "blastoderm") is 



formed. Formation of a gastrula furrow is a secondary phenomenon. Mid- 

 gut epithelium regarded as an ectoderm derivative in most Pterygota. Inner 

 layer wholly mesodermic Heymon's theory 



b. Gastrulation bipha.sed, the formation of thg gastrula furrow being the second 



phase. Inner layer with two components — lateral parts mesoderm, median 



parts (bipolar or with median strand or both) are secondary entoderm 



Hirschler's theory 



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



manner arising, is, in the majority of insects, a secondary phenomenon. It 

 may arise from (1) pure ectoderm (as in Locusta); (2) inner layer (i.e. from sec- 

 ondary entoderm, as in Carausnis); or (3) ectoderm plus secondar>^ yolk cells 



(secondary entoderm) RoonwaVs theory 



It is not out of harmony with any of these views to regard the definitive mid- 

 gut epithelium as being derived from primary yolk cells in the apteryotes 

 and the primitive pterj^gotes. 



2. Primary yolk cells nongerm-layer derivatives. The blastoderm stage represents 



the true blastula. Inner layer with two components, its lateral parts mesodermic. 



a. Primary entoderm buds off into the yolk from the inner wall of the ventral 



furrow but gives rise to no larval structures Mansour's theory 



b. Median part (primary entoderm) of inner layer confined to bipolar mesenteron 



rudiments, or to a mid-strand, or both. These parts give rise to the mid-gut 

 epithelium Extension of Kowalewsky's theory 



It should be noted that the terms " primary and secondary entoderm" 

 and "primary and secondary yolk cells" are used in different ways by the 

 various writers. Cholodkowsky (1891) and others employed the expres- 

 sion "primary entoderm" for the entire undifferentiated inner (lower) 

 layer whose components are mesoderm and "secondary" entoderm 

 (anterior and posterior enteron rudiments). The term "primary yolk 

 cell" usually signifies those cells which are left behind in the outward 

 migration of the cleavage cells during the formation of the blastoderm 

 (primary epithelium). The "secondary yolk cells " — those which return 

 into the yolk after the formation of the blastoderm — are either cleavage 

 cells that had reached the periphery and then turned back into the yolk or 

 cells formed by tangential division of the blastoderm cells. Tiegs and 

 Murray (1938) as well as other writers have taken exception to the 

 application of the term "entoderm" for the yolk cells unless these actually 

 give rise to the mid-gut epithelium. Spemann says that homologizing is 

 possible only after the formation of the anlagen, i.e., at the developmental 

 period when the individual parts of the germ band have become differ- 

 entiated, if not in their outward appearance, at least in their develop- 

 mental tendency. To this Tiegs and Murray add: "Homology of an 



