GASTRULATION, FORMATION OF GERM LAYERS 73 



ectodermal origin have adopted the view of Heider of a ''latent ento- 

 derm." Heider, contending that the mid-gut epithelium originates 

 from the entoderm, suggested that in the groups mentioned above, the 

 anlagen of stomodaeum and proctodaeum contained latent or dormant 

 entoderm cells. These latent cells later became distinctly differentiated. 

 This idea was more fully elaborated by the senior author (Johannsen, 

 1929). Tiegs and Murray (1938) object, however, saying that 



... it is evident that the notion of "latent entoderm" is in direct conflict with 

 the very essence of the (gastrulation) theory, viz., the occurrence of visibly dis- 

 tinguishable layers of cells in the very early embryo. 



In response to the last statement it may be asked what constitutes 

 "visibly distinguishable layers"? 



From time to time efforts have been made to reconcile the opposing 

 ideas. Heider (1897) quoting Braem tabulates three different views held 

 by morphologists relative to the homologies of the germ layers. (1) The 

 germ layers are purely topographical concepts, and they are solely deter- 

 mined by their relative positions. (2) The germ layers are homologous 

 in origin. (3) The gei-m layers are only analogous structures and only 

 comparable physiologically. They have, as Driesch says, the same 

 prospective significance. 



In most cases the discussions of embryologists have been based on the 

 second assumption that the germ layers are homologous in origin. The 

 first thing that is necessarj^ to reconcile the several theories is the deter- 

 mination of the status of the yolk cells. Are they to be recognized as 

 prim9,ry entoderm, or are they merely nongerm-layer derivatives? 



Because of their early differentiation the yolk cells were considered 

 by Dohrn (1866) and other early writers as the primary entoderm. They 

 believed that the mid-gut arose as a sac surrounding the yolk enclosed 

 anteriorly by the stomodaeum and posteriorly by the proctodaeum, its 

 walls being formed by the free cells or by cleavage. If the j^olk cells really 

 represented the entoderm, it was thought that with insects some repre- 

 sentatives, at least among the more primitive forms, would show this 

 type of development. Heymons (1897) in a brief paper on the develop- 

 ment of Lepisma and Campodea held that in these forms the yolk cells 

 play a part in the formation of the mid-gut epithelium, but according to 

 Tschuproff (1903) in Epitheca and Calopteryx the middle section of the 

 mid-gut originates from yolk cells. More recently Stuart (1935) has 

 described for Melanoplus differentialis the development of the definitive 

 mid-gut epithelium from yolk cells (vitellophags) which move peripherally 

 to form a lining upon the inner surface of the primitive mesenteron (mus- 

 cle and connective tissue components of the definitive mid-intestine). 



