NO. 3 EMBRYOLOGY OF FLEAS KESSEL 41 



formation of the blastoderm and which approach little by little and one after 

 the other to finally touch the blastoderm from underneath. The irregular form, 

 the largeness and the pale color of the nuclei of the first mesoderm cells show 

 a perfect resemblance to the primary entoderm cells on the one hand, and their 

 absolute dissimilarity regarding the same characteristics to the cells of the 

 ectoderm on the other. I believe, therefore, that the origin of the mesoderm 

 from the primary entoderm in Pulc.v serraticeps is beyond doubt. 



This quotation serves to illustrate the fact that the literature of 

 insect embryology contains many errors. Frequently, as in the present 

 case, the development of a certain form has been reinvestigated with 

 opposite results, causing one to doubt the accuracy of much that has 

 been written in this field. Such discrepancies are apparently due, in a 

 large measure, to the difficulty of the technique involved. Fragmentary 

 or otherwise poor sections prompt misinterpretations. The failure 

 of Tikhomirowa to observe the origin of the mesoderm from the 

 blastoderm may be ascribed to faulty preparations. It is a different 

 matter, however, when one considers her derivation of the entoderm 

 from the vitellophags. The origin of the cells which go to form the 

 mesenteron epithelium has been one of the most debated questions 

 concerned with insect development. In deriving the functional ento- 

 derm from the trophonuclei Tikhomirowa was merely following the 

 lead of numerous other investigators of her period. Their opinions 

 were doubtless motivated by a desire to harmonize the origin of the 

 germ layers in insects with the germ layer theory and with what was 

 known to occur in other groups. In the majority of animals the 

 entoderm is differentiated with the formation of the gastrocoel by 

 invagination. Consequently, in such forms the entoderm lies at the 

 interior of the egg. These workers expected, therefore, to find the 

 entoderm cells originating from the nuclei contained in the yolk. It 

 has been pointed out already that in other insects, as well as fleas, the 

 primary trophonuclei are augmented by cells migrating into the yolk 

 after the completion of the blastoderm. Dohrn and his followers, 

 seeing such cells intermediate between the vitellus and the surface 

 layer, probably mistook the direction of their progress and added this 

 observation to substantiate their belief. It may have been an attempt 

 to carry the germ layer theory one step farther that prompted Tik- 

 homirowa to derive the mesoderm from the same source as that from 

 which she supposed the entoderm originated. 



The mesenteron rudiments of flea embryos have been described in 

 the present paper as originating from near the anterior and posterior 

 extremities of the ventral plate blastoderm, making their initial appear- 

 ance before the formation of the lower layer is evident. Such a 



