THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 19 



Stage VII. oblique U -shaped stage — primitive streak is directed obliquely trans- 

 verse (Fig. 29). 



„ VIII. involute or 1/ -shaped stage — primitive streak directed backwards 



(Fig. 30). 

 „ IX. spiral or g-shaped stage (Fig. 33). 



„ X. biflexed or CO -shaped embryo (Fig. 35). 



„ XI. embryo with simple cephalic flexure or c — shaped embryo. 

 „ XII. pigmented embryo nearly ready for birth. 



Stage I. As mentioned above, the egg of P. novae-britanniae is small and without 

 yolk, and averages rather more than one-tenth of a millimetre in major diameter. 

 During the first two stages the egg-membrane is remarkably thick (007.5 mm.) and 

 must require special treatment in order to get the contained embryos properly pre- 

 served. In my sections through these stages they were all hopelessly collapsed, so 

 that I can give no details as to the process of segmentation. There are indications 

 however that up to a certain point the segmentation proceeds very much as in the 

 Neotropical species as described by Kennel and Sclater, and that it results in a solid 

 morula. But how the inner layer is formed I am quite unable to say. Most likely 

 it arises in situ in the solid morula as in the Mammalian ovum. 



Stage II. I have some preparations of embryos in the second of the above stages 

 where an oval cavity with sharply defined contour has appeared in the anterior portion 

 of the embryo, and apparently does not yet extend into the posterior third of the 

 embryo. At this stage the embryo measures "33 mm. 



Stage III. In Stage III. the embryo measures about 1 mm. in length. The 

 vitelline 1 membrane has become much thinner and consequently this is the first stage 

 of which I obtained adequately preserved representatives, capable of being mounted 

 in toto or of being cut into sections. I had two or three embryos at this stage, one 

 of which is shown in PL II, Fig. 16, and another in PI. Ill, Fig. 23. The embryonic 

 area proper is confined to a thickened tract at the posterior-ventral side of a large oval 

 vesicle. The rest of the wall of the vesicle is composed of embryonic ectoderm and 

 endoderm, which however take no immediate part in the formation of the embryo. 

 Physiologically it corresponds exactly with the peripheral epiblast and hypoblast of a 

 Mammalian blastodermic vesicle. As in the latter, it is the ectoderm which is chiefly 

 concerned in the absorption of nutriment for the use of the embryo as evidenced by the 

 vacuolar character of the cells. In view of this remarkable physiological resemblance of 

 this embryonic vesicle to the blastodermic vesicle of a Mammal we may well describe 

 it as a trophoblastic structure, adopting the word trophoblast in the sense in which 

 it has been employed by Hubrecht in relation to the peripheral epiblast of the 

 Mammalian embryo. (Hubrecht 9.) 



1 In this species vitelline membrane and egg membrane are used as synonymous terms. In P. capensis 

 according to Sedgwick and P. novae-zealandiae according to Sheldon there are two membranes, an outer firm 

 membrane and an inner more delicate membrane. The former is often spoken of as the egg-shell or egg- 

 membrane or chorion, and the latter as the vitelline membrane. 



3—2 



