316 Origin and ]\[igration of the Germ-Cells in Aeantliias 



tery, in the mesentery and finally dorsal to it, shifts by regular transi- 

 tions and furthermore the number at first close together becomes second- 

 arily scattered over a wide area and then contracted, but the cause of 

 ■this migration is somewhat difficult to determine. 



There are two views regarding this as well as a combination of the 

 two possible. 



First, they may migrate by independent amoeboid motions through 

 the cells that surround them. 



Second, they may migrate relatively to the embryo by themselves, 

 remaining comparatively fixed and many complicated changes in various 

 tissues of the embryo causing a shift of their position. 



It is easy to conceive how a growing together of the two lateral lips 

 of the blastodermic rim might convert Pig. 3 into Fig. 5, how a growth 

 of somatic cells into the ball together with an elongation and narrowing 

 of the embryo as a whole might bring about Fig. 7. 



Fig. 9 might be formed by a sinking of the intestine with reference 

 to the mesoderm combined with a segmenting of the mesoderm in a 

 more and more ventral direction, thus bringing some ova to lie in the 

 segmented mesoderm though none lay here in Fig. 7. 



In comparing the Figs. 7 and 9, it does not seem impossible that this 

 may be caused by a growth of tissue between the notochord and endo- 

 derm, together Avith a contracting down of the endoderm into a smaller 

 circumference. 



This would mean that the region between the primitive ova and the 

 top of the protovertebrre has not grown any in length from the condi- 

 tion in Fig. 5, where Ave see that the ova nearest to the top of the proto- 

 vertebrfB are fully as far away from the top as they are in Fig. 9 on the 

 right, or even about as far as they are in Fig. 11. 



It is to be remembered that Figs. 3-5, 7-9, 11-14 are all drawn with 

 a camera lucida Avith the same magnification so that the draAvings relative 

 to each other rej)resent the actual relative sizes of the embryos though 

 all are enlarged. 



The embryo during these changes has groAvn but little in height from 

 Fig. 3 to Fig. 11. It can easily be conceived hoAV an unfolding of the 

 tAvo lateral parts of the embryo in Fig. 5, combined Avith a splitting 

 of the mesoderm, Avith a sinking of the gut might produce Fig. 11, 

 adding, of course, certain other changes in the sizes of parts like the 

 enlargement of the notochord, spinal cord, etc. 



"We can reconcile Fig. 11, 8 mm., Avith Fig. 13, 15 mm., and not intro- 

 duce the question of independent amoeboid motion, if Ave suppose that 

 the mesothelium of the ccelom ventral to the mesentery and sin-rounding 



