MY RI APOD A 



407 



margins, antennal, mandibular, maxillary, postmaxillary (second maxil- 

 lary), and three double thoracic segments. Pflugf elder (1932) suggests 

 that the clypeus anlage may represent the preantennal segment, since it 

 later contains a coelomic sac. The preoral part is double, as indicated 

 by the presence of two pairs of coelomic sacs, but the division does not 

 extend to the lateral margins. Likewise, each thoracic segment is double, 

 as indicated by the presence of two pairs of coelomic sacs, although there 

 is but one pair of legs. 



sbm 



Fig. 363. — Platyrhacus amauros. Longitudinal section of gut. (ent) Entoderm cell, (lu) 

 Lumen of gut. (mes) Mesoderm, {stom) Stomodaeum. (y) Yolk. {From Pflugfelder.) 



At the time when segmentation of the body begins, it will be observed 

 that active proliferation of cells takes place at the point where the future 

 stomodaeum will form. These are the entoderm cells. Cells liberated 

 here migrate toward the posterior pole. The cells at the seat of prolifera- 

 tion become fewer and leave an opening at the surface that projects itself 

 into the proliferating cell mass to form a lumen (Fig. 363, lu). The 

 migrating cells are at first fusiform, then acquire large nuclei with large 

 nucleoli and relatively little chromatin material. Their plasma develops 

 long anastomosing processes which may entangle yolk globules, which, 

 however, are not found in the lumen. Gastrulation thus proceeds from 

 a polar migration accompanied by an invagination. After a certain time 

 when migrations of the entoderm cells cease, invaginations of the ectoderm 

 at the anterior and posterior poles occur, estabhshing the stomodaeum 

 and proctodaeum. The lengthening of these parts crowds the entoderm 



