OF PERIPATUS CAPENSIS. 907 



The first drawing (fig. 37), viz. that of the whole embryo, 

 shews an embryo of an oval shape, possessing six somites, 

 whilst along the middle of its ventral surface there are two slit- 

 like openings, lying parallel to the long axis of the body, and 

 placed one behind the other. The mesoblastic somites are ar- 

 ranged bilaterally in pairs, six on either side of these slits. The 

 following note in his handwriting is attached to this drawing: 



"Young larva of Pcripatus capcnsis. I could not make out 

 for certain which was the anterior end. Length i'34 milli- 

 metres." 



Balfour's three remaining drawings (figs. 40 42) are, as 

 already stated, representations of transverse sections of the 

 embryo figured by him as a whole. They tend to shew, as 

 he stated in the letter referred to above, that the mesoblast 

 originates as paired outgrowths from the hypoblast, and that 

 these outgrowths are formed near the junction of the hypoblast 

 with the epiblast at the lips of the blastopore. 



In fig. 42 the walls of the mesoblastic somites appear con- 

 tinuous with those of the mesenteron near the blastopore. 



In fig. 40, which is from a section a little in front of fig. 42, 

 the walls of the mesoblastic somites are independent of those of 

 the mesenteron. 



Fig. 41 is from a section made in front of the region of the 

 blastopore. 



In all the sections the epiblast lying over the somites is 

 thickened, while elsewhere it is formed of only one layer of 

 cells; and this thickening subsequently appears to give rise to 

 the nervous system. Balfour in his earlier investigations on 

 the present subject found in more advanced stages of the em- 

 bryo the nerve-cords still scarcely separated from the epiblast 1 . 



We have since found, in Balfour's material, embryos of a 

 slightly different age to that just described. Of these, three 

 (figs. 34, 35, 36) are younger, while one (fig. 38) is older than 

 Balfour's embryo. 



Stage A. The youngest (fig. 34) is of a slightly oval form, 

 and its greatest length is -48 mm. It possesses a blastopore, 



1 Comparative Embryology, original edition, Vol. I. p. 318. [This edition, Vol. n. 

 P- 385-] J 



582 



