Frederick Adams Woods 311 



mesoderm which will form the ccelom has^ however, not yet taken place. 

 At this stage most of the mesoderm and endoderm cells have altered 

 their original characteristics, being smaller and less clear in outline, 

 taking on spindle or cylinder shapes, losing more or less of the darkly- 

 stained yolk grannies and tending to fuse into a more mesenchyma-like 

 mass. 



Here and there, however, in the extra-embryonic mesoderm, endo- 

 derm and even in the yolk just beneath the endoderm, one finds numer- 

 ous cells of the original type. 



The appearance of such cells in the yolk just beneath the endoderm 

 and even frequently partly in the endoderm and partly in the yolk sug- 

 gests the idea that cells of the early type are being formed in the yolk 

 itself at the period during which the embryo undergoes its first two 

 or three millimeters of growth and that these cells contribute to the 

 formation of the endoderm and perhaps also to the Ureier. This con- 

 tribution of yolk cells to the endoderm i^ stated in Balfour. Or it may 

 be that the Ureier are derived solely from such yolk-formed cells. This 

 would give the Ureier an extra embryonic formation similar to the 

 blood corpuscles as claimed by His. " Angioblast," oo. 



"Whether the Ureier are formed in the yolk or in the endoderm, cells 

 of this type are now present in both and also in the mesoderm, but only 

 near its junction with the endoderm. 



It is important that so far none of the cells in the segmented meso- 

 derm or even near the 

 segmented mesoderm 

 have any such charac- 

 teristics. All cells of 

 regions are of the 

 small, dimly outlined 

 variety, carrying only a 



few volk granules. Fig. 3. Cross section, embryo of 2% mm. Ed. endoderm, 



' . tir.. germ-cells. A-B., location of all the germ-cells. Har- 



The next section vard Embryological Collection. Trans. Series 462, section 



(Fig. 3) IS taken 



through the hind segment of a 2^ mm. embryo with 9 somites. This is 

 through the heart of the primitive ova region and shows that these cells 

 are found far out on the yolk in the early stages. It is possible to count 

 them at this stage since all the remaining cells have changed their 

 characteristics in one way or another. There were 93 of them present 

 on the right side and all lay within the region marked A-B, most of 

 them being in the noticeable swelling caused by the union of the three 

 germ layers. Only 5 were in the mesoderm. 



