343 



S.N. 



In Fig. 2, this ridge of sex*cells is seen in transverse section to 

 be connected with the main body of endoderm by only a narrow neck 

 of cells. The lateral plates of mesoderm appear to be literally pinch- 

 ing it off. Fig. 3 shows this 

 region more highly magnified. 

 From a study of this section 

 one is led to the view that the 

 lateral plates of mesoderm play 

 a leading role in the migration 

 of the sex-cells, although such 

 is by no means the case in the 

 turtle. Another objection to 

 this view is found in the fact 

 that no sex -cells are thus 

 "pinched off' from the meso- 

 derm in a region extending 

 from the point where the Wolf- 

 fian ducts empty into the cloaca, 

 cephalad to the sex-gland an- 

 läge, a distance of 200 /.i, and 

 the same is true of the region 

 cephalad from the sex-gland an- 

 läge. This in spite of the fact 

 that in larvae younger than the 

 7.5 mm. stage the endodermal 

 ridge is as strongly developed 

 in these regions, as it is in the 

 intermediate region, 600 /^i in 

 length, which gives rise to the 

 sex - cells. Furthermore , the 

 mesentery is formed at the 

 same time in these regions as 

 in that of the sex-cell anläge, 

 but its two halves meet above 

 the endodermal ridge instead 

 of "pinching off" sex-cells from 

 it. This would tend to in- 

 dicate that the sex -cells differ from the neighboring cells of the 

 endoderm in having the power of independent movement which 

 enables them to pass up between the two approximating halves of 

 the mesentery. 



e.V. 



W.D. 



S.C.R. 



Arch. 



Fig. 2. Transverse section of a larva 

 of R. pipiens of 7.5 mm total length. W.D. 

 Wolffian duct. 



