THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRIT ANNIAE. 49 



Figs. 41—45. Stage IV. 



Fig. 41. Posterior region of embryonic tract behind the primitive streak. The section 

 involves the hinder end of the mesodermic proliferation. Shortly behind this region the 

 thickened cap-like plate of embryonic ectoderm ceases, and the wall of the trophic cavity 

 consists simply of trophic ectoderm and trophic endoderm. 



Fig. 42. Hinder portion of primitive streak (five sections intervening between this and 

 preceding). 



Fig. 43. Middle portion of primitive streak with shallow primitive groove. This is not 

 quite the deepest portion of the groove. 



Fig. 44. Middle region of the embryonic plate in front of the primitive streak, between 

 the latter and the cranial groove. The solid plate of mesoderm occupies the whole width of 

 the embryo between the thickened embryonic ectoderm and the endoderm. 



Fig. 45. Anterior region of embryonic plate. The mesodermic plate is here seen to be 

 dividing into paired mesodermic bands which are still united together across the middle line 

 by a narrow bridge. The separation of the mesoderm into paired bands coincides with the 

 depression caused by the deepening by the cranial groove. At this stage the stomodoeal 

 involution cannot be distinguished from the cranial groove. 



Figs. 46 — 50. Another embryo of the same stage, being the one shown in Fig. 25. 



Fig. 46. Posterior end of primitive streak. The mesodermic mass attains the surface of 

 the embryo and appears as a plug between the two portions of the embryonic ectoderm 

 which meet together behind this point. The embryonic vesicle is seen lying within the 

 chorion which is omitted from most of the drawings. 



Fig. 47. Middle region of primitive streak with groove. This is practically identical with 

 Fig. 43. 



Fig. 48. Anterior end of primitive streak. This section passes through the deepest part 

 of the primitive groove which forms at this place, at a later stage, the proctodoeal 

 involution. 



Fig. 49. Region of cranial groove. This groove extends over some seven or eight sections ; 

 its depth varies slightly at different points. 



Fig. 50. Anterior region of embryo in front of the cranial groove. A solid tongue of 

 ectoderm reaches the endoderm and completely divides the mesodermal bands from one another. 

 In comparison with the next stage this tongue may represent the delamination of the stomo- 

 doeum, or it may merely be related to the base of the cranial groove. In front of the 

 region represented by this section the embryonic plate rapidly dwindles out, and only the 

 trophic ectoderm and endoderm remain. 



Figs. 51 — 57. Stage VI. Sections through the embryo represented in Figs. 27 and 28. 



