ANATOMY AND DEVELOPMENT OP PERIPATUS OAPENSIS. 251 



that the mesoblast appears to originate as paired outgrowths 

 from the lips of the blastopore. 



The drawings left by Balfour in connection with the dis- 

 coveries are four in number : one of the entire embryo, show- 

 ing the slit-like blastopore and the mesoblastic somites, tlie 

 other three depicting the transverse sections of the same em- 

 bryo. 



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

 shows 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 arranged bilaterally in pairs, six on either side of these 

 slits. The following note in his handwriting is attached to this 

 drawing : 



"Young larva of Peripatus capensis. — I could not make 

 out for certain which was the anterior end. Length 184: 

 millimetres." 



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 show, 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. 40 the walls of the mesoblastic somites appear con- 

 tinuous with those of the mesenteron near the blastopore. 



In fig. 41, which is from a section a little in front of fig. 40, 

 the walls of the mesoblastic somites are independent of those 

 of the mesenteron. 



Fig. 42 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 



