IV PREFACE. 



My series of Balfouri embi'yos is complete up to and after 

 tliis period, and shows that the development does not differ 

 essentially from that of Capensis. 



When Balfour found the embryos of Peripatus Capensis 

 with an elongated and divided blastopore, it seemed probable 

 that the developm.ent would prove to be of more than usual 

 interest. This has I think been thoroughly borne out by the 

 investigation, the results of which are recorded in the following 

 pages. The more striking features of the development are 

 enumerated in the following statement : — 



The large size of the egg combined with the almost total 

 absence of yolk ; the character of the cleavage ; the structure 

 of the gastrula, and the mode of origin of the enteron ; the 

 division and persistence of the blastopore as the mouth and 

 anus ; the position of the primitive streak behind the anus and 

 its exact resemblance in structure to the primitive streak in 

 Vertebrata ; the presence of a pair of somites in front of the 

 mouth (in the preoral lobes) sending prolongations into the 

 antennae and developing the rudiments of nephridia; the origin 

 of the generative elements as nuclei in the endoderm which 

 subsequently migrate into the mesoderm ; the presence of the 

 nephridial end-sacs, and the coelomic nature of the generative 

 tubes; the persistence of the cerebral grooves as the sub- 

 cerebral bodies of the adult ; the origin of the lining of the 

 proctodseum and stomodseum from nuclei at the lips of the 

 blastopore intermediate between the nuclei of the ectoderm 

 and endoderm ; and lastly the presence of a pale subcutaneous 

 network from which the somatic muscles and nerves are pro- 

 bably developed in continuity with each other. 



Of the morphological significance of these facts this is not 

 the place to speak, but I may perhaps draw attention to two 

 subjects of a general interest which are touched upon by my 

 investigations. 



