STRUCTURE OF ONYCHOPHORAN HEAD — BUTT 53 



muscles originating caudad of 13 on the body wall but below the slime 

 gland duct and inserted on the retractor apodeme. 



Such evidence concerning the homologies of the feeding claws as 

 given above would be inconclusive were it not supported by the evi- 

 dence of embryonic development. Here, too, investigators who have 

 worked on the development of this form are in disagreement. Un- 

 fortunately no material was available for a study of this kind. Conse- 

 quently one must rely on the works of others for information on head 

 development. Of those who have published on the embryology of 

 Peripatus, Pflugfelder (1948) and Manton (1949) have worked most 

 recently, and it is mainly from their papers that the following account 

 has been taken. 



DEVELOPMENT OF THE EMBRYO 



The entoderm and mesoderm of Paraperipatus amboinensis originate 

 separately from cells proliferating inwardly from the ventral part of 

 the blastoderm according to Pflugfelder. The entoderm appears first 

 and forms in a short time an inner lining which becomes one cell in 

 thickness except where the cells bunch up at the zone of proliferation 

 (fig. 4 E, Ent). This point of proliferation is flanked by tall, slender 

 cells known as fibroid cells {Fz). A groove forms beneath this zone 

 in the early gastrula stage which Pflugfelder calls the primitive groove. 

 He purposely avoids the term "blastopore," for he says, "such a porus 

 appears nowhere during the fetal development of Paraperipatus am- 

 hoinensis," This groove apparently corresponds to the early transient 

 blastopore noted by Manton. The development of the endoderm con- 

 tinues long after the majority of coelomic pouches have appeared (fig. 

 4G). 



The formation of the mesoderm, like the formation of the entoderm, 

 takes place at a spot closely behind the primitive cavity, being formed 

 from the beginning in pairs. The right and left immigrating zones 

 are separated by median fibrous cells (fig. 4 F). At the points of 

 proliferation, the cells accumulate as domelike invaginations, but 

 laterally they thin out into single cell layers which push between the 

 entoderm and the ectoderm (fig. 4 F, Mes {lat) ) . Even in these dome- 

 like masses the cells have a tendency to arrange themselves into a 

 single layer which causes the coelomic cavities to form (Coel). 



An undifferentiated mesodermal band does not appear in the head 

 region in front of the primitive cavity; the immigrated mesodermal 

 material rather differentiates close to its place of origin into the paired 

 coelomic pouches and into the further proliferating lateral mesoderm. 



