STRUCTURE OF ONYCIIOPHORAN HEAD BUTT 55 



Through the longitudinal growth of the ectoderm the coelomic 

 pouches are passively removed from the place of their origin ; i.e., 

 from the very beginning they remain connected with that point of the 

 germ band to which they belong functionally, according to Pflugfelder. 



In some species the blastopore and thus also the point of prolifera- 

 tion of the mesoderm remain at the posterior end of the body. Ac- 

 cording to Manton, at the stage when the germ band forms on the 

 surface of the blastoderm, two germinal discs occur, in various species 

 of Peripatopsis worked on by her, as separate thickenings of the blas- 

 toderm. The posterior thickening (Pth) arises first, followed quickly 

 by an anterior thickening (fig. 4 A, B, Ath). The posterior thickening 

 {Pth) gives rise to the blastoporal area from which the mesoderm is 

 formed in all species described by her, and from which in some species 

 the entoderm is also formed. The anterior thickening gives rise to the 

 ectodermal part of the lips of the mouth-anus (fig. 4 A, B, Ath) and 

 later to the midventral ectoderm of the body. 



The proliferating mesoderm forms a U, with the arms pushing in 

 an anterior direction around the anterior thickening (fig. 4 C, Mes). 

 When the arms reach the halfway mark along the mouth-anus, the 

 anterior portions of each arm break away, become hollow, and form 

 the coelomic sac of the first somite (fig. 4 C, Coel i). As the arms 

 continue to push forward, succeeding somites are formed in a like 

 manner. In the meantime the groove forming the mouth-anus has 

 elongated and the middle edges have grown together, leaving only 

 the mouth and anal openings which become ever more widely sepa- 

 rated as the embryo grows in length. Figure 4 D shows the embryo 

 with the last pair of coelomic sacs separating from the mesodermal 

 band, after which the mesodermal band soon disappears. 



Manton says of the anterior sacs, "The first pair approach each 

 other anterior to the mouth and establish the antennal segment. The 

 second pair are smaller and lie at the side of the mouth where they 

 establish the mandibular segment. The third pair are larger than the 

 second and all succeeding somites in early stages in some species . . . 

 and establish the slime papilla segment." 



As for the development of the nerve cord we must turn to Sedg- 

 wick (1885), Kennel (1888), and to Pfiugfelder, since Manton does 

 not discuss this important phase of development in her paper. The 

 nerve cord of Paraperipatus amboinensis according to Pflugfelder 

 develops from paired thickenings on the ventral side of the embryo. 



These ridges become segmented as the result of concentrations of 

 ganglion cells, and the resultant lobes are known as the ventral organs. 



