THE FORMATION OF THE GERM-LAYERS. 317 



in Melolontha), these later immigrated cells are said to be clearly 

 distinguishable by their histological character from the cells origin- 

 ally found in the yolk. 



The yolk-cells are distributed in a regular manner through the 

 food-yolk. Their principal function appears to be digestive, particles 

 of food-yolk being taken up by these cells and changed in such a 

 way as to render the yolk assimilable by the growing cells of the 

 embryo. This leads, after the development of the germ-band is 

 completed, to the marking-off of the territories belonging to the 

 individual yolk-cells, and this process has been described as secondary 

 yolk-cleavage (Fig. 158 C-F, p. 321; Fig. 135, p. 273). In indi- 

 vidual cases (Apis, Mused) such cleavage, however, seems not to 

 occur. The yolk -cells can still be recognised in the completely 

 developed enteron in the remains of food-yolk which fill it, and 

 here they gradually disintegrate. 



It was long considered by followers of Doiirn, Balfour, and Hertwig that 

 the yolk-cells represented the actual entoderm of the Insecta, as it was thought 

 that these cells finally became arranged at the surface of the food -yolk to form 

 the enteric epithelium. This view has to be relinquished in face of the more 

 recent researches, on which the account of the formation of the germ-la3 T ers 

 given above is founded. It appears that the yolk-cells do not in any way 

 take part in the formation of the embryo. It was indeed suggested in several 

 quarters that they gave rise finally to blood-corpuscles or parts of the fat-body 

 (Dohrn, No. 21, Tichomiroff, No. 79, and especially Will, No. 97). A 

 number of more recent authors, however, oppose this vieAV, and maintain that 

 the yolk-cells, after having fulfilled their function as vitellophags, simply 

 disintegrate. This last view seems to us the most probable, since another 

 origin has been proved for the fat-body and the blood-corpuscles (p. 341). 



Bearing in mind the statements made above in connection with the Crustacea 

 (Vol. ii., p. Hi), we may probably regard the yolk-cells as an abortive portion 

 of the entoderm. 



[Recent observations have once more rendered uncertain the origin of the 

 mesoderm, the nature of the epithelium lining the alimentary canal, and the 

 true significance of the primitive groove. Thus Heymoxs (Nos. XV. and XX.), 

 states that in the Orthoptera, the ento-mesoderm of other authors is to be 

 regarded as consisting of mesoderm only, the lining of the definitive alimentary 

 canal arising from the ectodermal epithelium of the stomodaeum and procto- 

 daeum. He further states that the primitive groove (blastopore of authors) 

 may be completely wanting, and even when present is not to be regarded as 

 connected with gastrulation. Lecaillon (No. XXIX.) finds that in the Chryso- 

 mclidac the whole alimentary canal is ectodermal. These two authors think 

 that the higher Insects exhibit no entoderm in the alimentary canal of the 

 adult, while in the lower forms (Heymoxs, No. XVI., Lepisma) the enteron arises 

 from the yolk-cells. On the other hand, Burger and Careiere (No.. II.), with 

 whom Wheeler agrees, are fully convinced that a true enteron exists in 

 Chalicodoma, and entirely dissent from Heymoxs' views. They show that the 

 entoderm arises from the undifferentiated blastoderm, and that the stomodaeal 

 and proctodaeal invaginations arise from the superficial layer of blastoderm- 

 cells, the only layer that can properly be called ectoderm. See also Heiher 

 (No. X.).-En.] 



