1907.] 



NATURAL SCIENCES OF PHILADELPHIA. 



53^ 



relation of those cells in the liiugramatic optical section (text fig. 2). 

 Text fig. 3 represents an actual optical section of a somewhat later 

 stage. Here the large nuclei of 4a, 4b, and 4c are again shown, while 

 lying just above them is a group of mesotlerm cells derived from the 

 division of -icF. The mesoderm now consist of a large number of cells. 

 From this time on these begin to spread out and gradually form a 

 layer of cells lying just beneath the ectoderm. 



While 1 have not been able to follow the exact lineage of these cells, 

 I think there is no doubt that the greater portion of the mesoderm of 

 the body arises from the posterior mesoblasts, while as stated before 

 only the mesodermal structures in the region of the pharynx arise 

 from cells of the second quartet. 



Mies 2 



mes. I 



end. 



mes .9 



tndc 



Fig. 3. -Optical section of an egjz; viewed from the posterior side, end, Group 

 of endoderm cells derived from 4<:/^ Below these are the degenerating 

 nuclei of the four niacronieres. Near the middle of the egg are the three 

 large nuclei of 4o, Ah and 4r. Above these are shown the nuclei (stipled) 

 of a few of the mesoderm cells derived from A(P, mes. 1. Toward the vegeta- 

 tive pole are mesoderm cells derived from the second quartet, mes. 2. 



Text fig. 3 shows a number of cells lying further towards the vegeta- 

 tive pole than the derivatives of 4cP have yet reached. These cells 

 undoubtedly arise from the further proliferation of the second quartet 

 cells, 2a*-*-^-2(Z**-^, which, as we have shown (p. 535), were budded into 

 the interior of the embryo (cf. fig. 32). These second quartet cells, 

 2a' ■^•^-2f/^-*-^, when first formed lie considerably below the equator of 

 the egg. With the overgrowth of the ectoderm they are carried 



