FOEMATION OF THE MESODERM. 39 



the polar area has a well-marked groove (PI. IVj fig. 22 a) 

 which is the primitive groove. 



Formation of the Mesoderm. 



The nuclei of the mesoderm are derived from the nuclei of 

 the polar area. The latter increase largely in number (PI. 

 V, fig. 25 b) and form a primitive streak. An early stage 

 of this process is shown in figs. 22 a, b. It begins at the 

 front end of the area, but soon the nuclei of the whole area 

 are implicated. They are constantly met with in a state of 

 division. 



In the next stage, stage a, figured on PL I, fig. 22, a well- 

 marked primitive streak is visible when the embryo is examined 

 from the surface. 



A series of sections through such an embryo (PI. V, figs, 26 

 a — d) show that the blastopore is still traversed by a reticulum 

 (figs. 26 a, b), and that the primitive streak is largely developed 

 (figs. 26 c, d), and its front part traversed by a well-marked 

 groove. In the deeper parts of the primitive streak, at about 

 the middle of its length, there is an area of protoplasm con- 

 taining two (perhaps more) nuclei, and characterised by the 

 relative predominence of the extra-nuclear protoplasm. This 

 area is shown in section in fig. 26 d. I cannot help thinking 

 that it is derived directly from the hinder part of the polar 

 area of the previous stage figured in PI. IV, fig. 21, and 

 PI. V, fig. ,24 d. It seems to me that while the nuclei of 

 the polar area on each side of this structure constantly undergo 

 division (fig. 22 c, 24 d) the nuclei in this structure do not 

 divide, but that it becomes overgrown ventrally by the proli- 

 ferating lateral nuclei of the polar area (PI. IV, fig. 21), and 

 thus comes to acquire a deeper position. This would seem to 

 imply that the growth of the mesodermal nuclei in the hinder 

 part of the polar area is a bilateral process, that the cells on 

 each side of the middle line only proliferate ; and I think that 

 a careful examination of the anterior part of the polar area 



