46 THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 



call for special notice are the caudal region of the embryo, the first pair of somites which have 

 increased greatly in size, and lastly the relation of the embryo to the trophic vesicle. 

 Embryo from specimen IV. Total length 4-25 mm. 



Fig. 29 a. Another embryo of the same stage with primitive streak directed quite trans- 

 versely and so concealing all the somites, except those of the first pair, which occupy the pro- 

 cephalic lobes. This embryo immediately succeeded that shown in Fig. 27 ; when lying in the 

 uterus the posterior end of its trophic vesicle abutted on the anterior end of the latter. 

 Chorion omitted. From specimen II. Total length about 5 mm. 



Fig. 30. Stage VIII. Frontal view. The caudal end of the embryo has grown to such 

 an extent as to cause a double flexure in the embryo whereby the region of the primitive 

 streak becomes directed backwards and a partial restoration of symmetry is the result. In 

 embryos at this and the preceding stage, the two regions which can always be definitely 

 recognised are the anterior and posterior extremities of the embryo. This is due to the 

 characteristic shape of the caudal end and to the large size of the procephalic lobes with their 

 contained somites of the first pair. Embryo from specimen II. Total length about 5 mm. 

 Chorion omitted. 



Fig. 31. Enlarged view of the same embryo from the left side. It is clearly shown here 

 how the trophic vesicle occupies the dorsum of the embryo, and is therefore essentially a dorsal 

 structure. The dorsal sides of the procephalic lobes are confluent with the wall of the 

 vesicle. The margin of the body is lobed by the outgrowing appendages, only those of the 

 left side of the embryo being seen, except that the right procephalic lobe shows through 

 from the other side by transparency. The lobe, which closely hugs the left procephalic lobe, is 

 the future manducatory appendage of the left side. This is followed by the future oral 

 papilla of the same side. These two lobes can already be distinguished from the succeeding 

 appendicular lobes. The recurved caudal region can be seen by focussing down, and it is in- 

 dicated by a dotted line. 



Fig. 32. Embryo belonging approximately to the same stage as preceding. It lies in an 

 acute bend of the uterus. The anterior portion of the trophic vesicle, or head-fold, is con- 

 siderably larger than the posterior portion, or tail-fold. The latter has apparently shrunk 

 away somewhat from the chorion. From specimen VII. As in other species, so in P. novae- 

 britanniae, the uterus is constricted at intervals, but the intervals are not regular and the 

 constrictions are not always adapted to the position of the ernbryo. It is not uncommon to 

 find an embryo tightly squeezed by a uterine constriction. On the other hand, it is still more 

 common to find two or more embryos placed end to end without any intervening constriction. 



Fig. 33. Stage IX. The caudal and abdominal regions have now increased so much in 

 their independent growth that the caudal extremity lies in front of the head. It is only 

 the cephalic half of the embryo which is in direct continuity with the trophic vesicle. All 

 that portion of the posterior part of the body which in the figure is seen to lie in front 

 of the head is free from the trophic vesicle and only lying upon it as on a cushion. 

 The cephalic extremity, with the procephalic lobes from which the antennae have begun to 

 bud out, still occupies its primary position, and, when lying in the uterus, points in the 

 direction of the vagina. The small circles, some distance behind the antennae, represent the 

 manducatory appendages showing through by transparency. Chorion omitted. Total length 

 upwards of 4 mm. Although the total length of the embryonic vesicle is somewhat reduced in 

 this stage, the size of the embryo itself is much greater. This is a natural consequence of 



