DEVELOPMENT OF THE CAPE SPECIES OP PEEIPATUS. 473 



nucleated protoplasm derived from the superficial layer of the 

 preceding stage. The hump, however, still persists, retaining 

 the characters it had in Stage e. 



With regard to the internal boundary of the ectoderm, in 

 the gastrula stage there was no line of demarcation between 

 it and the endoderm. In Stage b the mesoderm appears, 

 but causes no break in the continuity (PI. XXXIV, fig. 

 5 a — -/). In Stages c and d, however, a definite separa- 

 tion occurs, firstly by the appearance of the cavity in the 

 somites, and secondly — and this happens later, in Stage 

 D — by the dorsal and ventral separation of the endo- 

 derm from the ectoderm. The endoderm is, however, still 

 continuous with the splanchnic layer of mesoderm, and the 

 ectoderm with the somatic. In the subsequent development 

 this continuity seems to be retained and to be extended in 

 consequence of the growth of the mesoderm over the internal 

 surfaces of the at first uncovered parts of the two primary 

 layers. At any rate I have never been able to see any well- 

 defined boundary between the layers in question, even in the 

 best-preserved embryos, if a careful examination was made 

 with a high power. The defined line drawn in my figures has 

 only an existence with a low power -, it is therefore extremely 

 difficult to say whether or no nuclei pass in from the ectoderm 

 to the mesoderm, and often not possible to settle for certain 

 whether a given mass of nuclei belong to the ectoderm or 

 somatic mesoderm. In the later stages (r) this absence of a 

 defined line continues, so that it becomes difficult to decide 

 whether the external transverse muscles of the body wall are 

 ectodermal or mesodermal. To this point I shall return in 

 Part IV of this series. 



2. The lateral thickenings, which give rise, amongst 

 other things, to the nervous system, contain during Stages a 

 and B a single layer of long oval nuclei (PI. XXXIV, fig. 5 5). 

 They are at first confined to the ectoderm immediately over- 

 lying the mesoblastic somites, and constitute on each side a 

 continuous band extending from the pre-oral region, where 

 they are more ventrally placed and continuous with each other 



