THE MAI^AYAN SPECIES OF ONYCHOPnOIlA. 57 



native is more probable^ because in the embryo vmder con- 

 sideration tliey occupy a central as well as a peripheral 

 position in the endoderu). If tin's supposition be correct, 

 their formation from tlie spherical masses of deutoplasm, 

 which occur in the endoderm of the third embryo, would 

 have to be explained on the same principle as the building 

 of compound systems from separate masses of deutoplasm 

 in the case of the ovarian ovum. In the third place, the re- 

 fringent granules found in the third embi-yo do not seem to 

 occur in the present one. In the fourth place, nearly' all the 

 endodermal nuclei display an angular outline ; while in the 

 tliird. embryo only a few of these, situated in the central 

 endoderm masses alone, presented the characteristic in ques- 

 tion. In the fifth place, the endodermal cytoplasm, apart 

 from the difference arising from the change in character of 

 the deutoplasm, is quite different in structure, in that in 

 many places it appears distinctly fibrous, the fibres being- 

 arranged according to no particular plan (PI. 7, hg. 12 e). 



V. The Development of the Mesodermal Organs. 



In the foregoing pages an attempt has been made to 

 describe the formation of the Germ Layers, and to give a 

 brief account of what Inippens during the early stages of the 

 development; but in the following pages it will be my en- 

 deavour to give a more detailed account of the development 

 of the Organs from the Germ Layers. 



(1) The Development of the Mesoderm with its 

 Cavities. 



The development of the mesodermal somites has been 

 followed both by von Kennel (4) and by Mr. Sedgwick (5). 

 The description given by the former of their development in 

 Peripatus edwardsii is so unsatisfactory as to deserve no 

 further mention in this connection ; while that given by the 

 latter of the processes going on in Peripatopsis capensis 

 is so good that, even now, after an interval of fifteen years 



