THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 45 



PLATE III. 



Fig. 23. Stage III in side view and optical section. The embryo is enclosed within 

 the chorion (or egg-membrane) and consists of a large oval vesicle, the trophic vesicle, the 

 wall of which is of uniform thickness except at the posterior ventral end of the vesicle 

 where there is a much thickened area, the embryonic tract. Actual length of entire embryonic 

 vesicle about 1 mm. 



Flu. 24. Posterior portion of an embryonic vesicle at a stage intermediate between 

 Stage III and IV, in side view and optical section. Chorion omitted. The embryonic tract 

 no longer abuts upon the posterior extremity of the vesicle, since the latter has begun to 

 grow backwards to form the caudal extension of the trophic vesicle. Total length 1'45 mm . 

 This embryo succeeded the preceding in the same uterus (specimen IV). 



Fig. 25. Stage IV. Portion of embryonic vesicle with embryonic tract, from the left 

 side, showing the primitive (posterior) and cranial (anterior) grooves. The portion between 

 these grooves, in comparison with the next stage, occupies the position of the future ventral 

 surface of the embryo. Apparently the invagination which occurs in the next stage to form 

 the definitive ventral surface proceeds from, or at least involves, the deeper posterior portion 

 of the cranial groove. This embryo was taken from the same individual (VII) to which 

 Fig. 16, Plate II relates. Total length 2 7") nun. Chorion omitted. 



Fig. 26. Stage V. Similar view showing appearance of a transverse groove in the 

 middle of embryonic tract. This is the commencement of the formation of the ventral surface, 

 or, in other words, it is the commencement of the ventral flexure, in situ, of the embryo. 

 The embryo is not seen strictly in side view but in \ side view, so that the cranial groove 

 which separates the procephalic lobes is to be seen. This embryo comes from the same individual 

 (IV) as those represented in Figs. 23 and 24. I have a permanent preparation in Canada 

 balsam of this embryo. Chorion omitted. 



Fig. 27. Stage VI. Frontal view of an embryo in which the transverse groove seen 

 in the preceding stage has taken a crescentic form owing to the forward free growth of 

 the primitive streak. The latter now projects forwards like a tongue. The anterior portion 

 of the trophic vesicle, or the trophic head-fold, is more extensive than the posterior portion, 

 or trophic tail-fold. Embryo from specimen II. Total length 3-25 mm. Chorion omitted. 



Fig. 28. Enlarged frontal view of the embryonic tract of the same embryo. The centre 

 of that portion of the embryo proper, which lies in front of the crescentic groove, is occupied 

 by the cranial groove which is bounded by the first pair of somites, the outlines of which 

 can be seen by transparency. The somites of the second pair lie at the outer sides of those 

 of the first pair, while those of the third pair occupy a similar position with regard to 

 those of the second pair. The dotted line indicates the plane through which the section 

 represented in Fig. 51 was taken. 



Fig. 29. Stage VII. At this stage the primitive streak or caudal end of the embryo has 

 by its continued growth become directed oblique-transversely, thus occasioning a superficial 

 asymmetry in the entire embryo. The crescentic form of the embryonic tract, which is 

 characteristic of the preceding stage, is still discernible. Owing to its oblique growth it is 

 very difficult to describe the embryo in detail at this stage, and I have not attempted it. 

 The overlapping of the mesodermal somites causes great complication. The features which 



