40 SEGMENTATION AND FORMATION OF THE LAYERS. 



shows that the growth of nuclei there also is a bilateral one, 

 though the bilateral nature of the growth is not so obvious as 

 it is behind. The reason of this is that behind there is a 

 median structure — the hinder part of the polar area with its 

 round nuclei — on each side of which the growth appears to 

 take place, while in front there is no such well-marked 

 median structure, but there is the groove ; and I think that a 

 careful examination of the relation of the growing nuclei to 

 this groove shows the bilateral nature of the growth. I refer 

 in support of this to figs. 32 a, 25 b, 26 c, which are all 

 sections through the front part of the primitive streak, fig. 22 a, 

 being of course from the youngest of the embryos; and to 

 figs. 25 a, 26 b, which are in each case the last section through 

 the blastopore. It is difficult to say whether 25 a is to be 

 regarded as passing through the hind end of the blastopore or 

 through the front end of the streak, but it presents nuclei, 

 which must be regarded as mesodermal, in a position which is 

 consistent with the view that they have been derived from the 

 row of nuclei which extend between the ectodermal and endo- 

 dermal nuclei. 



Again, in fig. 26 b, we see similarly placed nuclei in the 

 act of division, with what must be regarded as mesodermal 

 nuclei on their inner borders. 



Further back (figs. 22 a, 25 b, 26 b) the blastopore is re- 

 presented only by the groove, and it is more difficult to satisfy 

 oneself on the point. 



However, I am inclined to think that the growth of primitive 

 streak nuclei is a bilateral one, in the anterior as well as in the 

 posterior part of the primitive streak, though I admit that the 

 evidence in favour of this view is not entirely satisfactory. 



If I am correct in this supposition, and in my conjecture 

 that the primitive groove is a rudimentary posterior part of the 

 blastopore (it is so considered in other tracheate embryos), 

 then the development of the mesoderm in Peripatus consists in 

 an ingrowth of nuclei from the lips of the blastopore and 

 resembles that described in so many other forms. 



The mesodermal nuclei of the primitive streak now grow 



