THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 21 



in PL III, Fig. 25, and will, I hope, suffice to elucidate the structure of the embryo at 

 this stage. As these figures are described as fully as possible in the explanation of the 

 plates at the end of this memoir, I think it will not be necessary to repeat here what 

 is stated there. 



Stage V. (PI. Ill, Fig. 26). In this stage a transverse groove has formed across 

 the centre of the embryonic area, and pari passu with the appearance of the groove, 

 the ectoderm underlying the groove has become a very thin layer with nuclei arranged 

 in a single row. In the rest of the embryo and in the whole of the embryonic plate 

 of the foregoing stages, the nuclei of the ectoderm are arranged in multiple rows. This 

 transverse groove, which is accompanied by local thinning out or flattening of the 

 ectoderm, is the Anlage of the ventral surface of the animal. It finally separates the 

 cephalic end from the caudal end. The embryo is now to all intents and purposes bent 

 double upon itself, but the doubling up is a passive procedure and is effected in situ 

 by the involution which gives rise to the ventral surface. The flexed embryo is pro- 

 duced, as I have just said, in situ, and a somewhat similar method of development in 

 situ has been described by Miss Sheldon in P. mmae-zetila adiae where, as is known, the 

 egg is of large size and contains abundant yolk. 



Stage VI. (PL III, Figs. 27 — 28). I had one very satisfactory embryo belonging to 

 this stage, and it is faithfully portrayed from the frontal aspect in Figure 27. The 

 total length of the cylindrical blastodermic vesicle, which is now proportionately at its 

 maximum development, was 3*25 mm. It will be noticed how small a tract of this 

 enormous trophoblastic organ is occupied by the embryo proper. It is from the attentive 

 examination of such an embryo as this that one may obtain the best impression of 

 the very singular mode of nutrition of the embryo of P. novae-britanniae. 



The ventral transverse groove now appears crescentic in shape in surface view. 

 This effect is due to the growth of the primitive streak which becomes raised up from 

 the surface of the vesicle and projects forwards, arching over the ventral surface. As 

 in all other cases where it occurs, the primitive streak is here essentially the growing 

 point of the embryo. It consists of a solid undifferentiated mass of cells which by 

 their remarkable power of proliferation cause the caudal end of the embryo to twist and 

 turn in the manner characteristic for this species. Thus the anterior region of the 

 embryo is practically a punctum fixum, and the contortion of the embryo in a later 

 stage is almost entirely due to the growth which is taking place at the primitive streak. 

 At this stage the free growth of the latter has already commenced but the embryo is 

 still symmetrical, and that is why it is so instructive. It cost me a struggle to cut 

 this unique embryo up into sections, but it had to be done and the result was satisfactory. 

 So well were these embryos preserved in the formol solution which I employed, that 

 mitotic figures are frequently met with in the mesoderm. In surface view the somites 

 of the anterior region were distinctly visible, and the first three pairs of somites can be 

 seen in Fig. 28. From the figure referred to it will be at once evident that a single 

 transverse section may involve several pairs of somites. For at this stage, as in the 

 preceding stage, the transverse diameter of the embryo proper is nearly twice the length 

 of its antero-posterior axis. 



