NO. 3 EMBRYOLOGY OF FLEAS KESSEL 39 



to the method by which the cells of the lower layer reach their sub- 

 merged positions. In the anterior region of the embryonic rudiment 

 where the cells sink by an irregular migration, this lateral progress is 

 likewise by an apparently haphazard movement (pi. 9, fig. 67). In the 

 section where the shallow groove is formed and in those regions of 

 it in which the mesoderm-forming cells migrate in parallel columns, 

 these lines of cells separate sharply at the lower level midline and pass 

 in opposite directions toward the lateral limits of the germ band 

 (pi. 8, fig. 64; pi. 9, fig. 69). This is in agreement with the observa- 

 tions of Patten ( 1884) on Ncophylax. In connection with that portion 

 of the embryonic rudiment where a distinct mesodermal tube is 

 formed, the beginning of the process is identical with that in which 

 no groove is formed. The first cells to leave the surface pass inward 

 with no space between them. The invagination takes place above this 

 solid mass so that the lower wall of the tube, when it is completed, is 

 two cells thick (pi. 8, fig. 65). With the compression of the tube 

 to obscure its lumen, these lower cells separate and migrate laterally 

 to form the extreme marginal parts of the unproliferated lower layer. 

 Figure 66 (pi. 8) shows this stage in a section cut at a level slightly 

 posterior to that from which the one shown in figure 65 was made. 



In the three forms of lower layer formation which occur in the 

 development of the flea embryo, it is evident that the first cells to 

 leave the surface tend to form the most lateral portions of the meso- 

 derm. The relative positions of the cells in the lower layer with 

 respect to the midline are obviously the reverse to what they were 

 when these same cells were still a part of the blastoderm. This situa- 

 tion is the direct opposite of that occurring in Apis as described by 

 Kowalevsky ( 1871 ) and Nelson (1915). Although the lower layer, 

 when first formed in fleas, is typically only one cell in depth, it is 

 soon thickened by cellular proliferation. This is shown by the mitotic 

 figures which are evident among the mesoderm cells. 



Of the five workers who have written on the embryology of fleas, 

 only those two who used the section method of study have mentioned 

 the germ layers. These are Tikhomirowa (1892) and Strindberg 

 ( 1917). However, the statements of both of them in this connection 

 are inaccurate. In respect to the origin of the lower layer ( mesoderm) 

 Strindberg omits its consideration entirely, confining his remarks to 

 the mesenteron rudiments. He merely states that in Archaeopsylla 

 erinacei the epithelium of the mesenteron is derived from one anterior 

 and one posterior anlage originating from the lower layer. Since the 

 mesenteron rudiments of the forms investigated during this study 



