Frederick Adams Woods 



313 



with low powers of the microscope and should be included in a descrip- 

 tion of the gross anatomy of the part. As yet there are no cells of this 

 type higher np in the mesoderm. 



The above section, Fig. G, was taken at the junction of the three 

 germ layers in the posterior region of another embryo of 3^ mm. It 

 will be seen that the ball cluster is broken up and also that the cells 

 are found at some little distance up into the mesoderm. On one side 

 in this embryo there were 230 cells of the primitive ova type, all very 

 uear together in this part of the body. 



Another section, Fig. 7, shows the position of most. of the Ureier in 

 a -J: mm. embryo. These clusters of primitive ova were mentioned by 

 Balfour who observed them in the genital regions of older embryos and 

 considered them due to rapid cell division. I have never seen, during 

 these earlv stages, anv indications of cell division. It would seem that 



-En 



Fig. 7. Cross section embiTO i mm. En. endoderm, Ur. germ-cells. Harvard 

 Embryological Collection. Trans. Series 464, section 123. x 38. 



Fig. S. Embryo .5 mm. En. endoderm, Ur. germ-cells. Harvard Embryological 

 Collection. Trans. Series 331, Section 27.5. x 38. 



Fig. 9. Embryo 6 mm. En. endoderm, Ur. germ-cells. Harvard Embryological 

 Collection. Trans. Series 293, section 31S. x 38. 



these clusters or egg nests were here merely due to the fact that a little 

 earlier, they practically all lie in two groups, one on each side in the 

 narrow mesoderm and that in such a drawing as Fig. 8 they have not 

 yet all wandered away from each other. 



Fig. 8 shows the principal location of the cells in a 5 mm. embryo. 

 Fig. 9 is of a 6 mm. embryo. One cell on the right is indicated as hav- 

 ing reached the segmented mesoderm. Up to this point the mesoderm 

 has not split. The next section. Fig. 11. embryo of 8 mm. shows the 

 split in the mesoderm to form the body cavity. The primitive ova 

 practically all succeed in getting on the splanchnopleure or inner layer; 

 just how is, to me, a mystery. 



A few are now in the genital region or dorsal end of the ca-lom, but 

 most of them are in the epithelium all along around the intestine and 

 mesenterv. 



