472 ADAM SEDGWICK. 



Stage E, it undergoes a striking change. It becomes much 

 thickened. The thickening first appears in Stage d in the region 

 of somites 7 — 10 (fig. 25), where indeed it lasts longer and is 

 more marked than elsewhere, and gives rise to the prominence 

 marked d in the figures of PI. XXXII, Part I. The change 

 begins at the sides and gradually extends dorsalwards. The ecto- 

 dermal hump (d.) seems to retain until its disappearance indi- 

 cations of this bilateral origin. The thickness of the dorsal 

 ectoderm varies in different specimens, and no doubt depends 

 to a certain extent on the amount of contraction which the 

 protoplasm has undergone at death. 



This increase in thickness is mainly due to the appearance, out- 

 side the nuclei, of a layer of vacuolated protoplasm. Thevacuola- 

 tion is not shown in my figures, but it is a very striking feature. 

 The surface of the dorsal ectoderm, particularly of the hump, 

 is very rough in these stages, and in the best-preserved em- 

 bryos without a definite external boundary. It presents very 

 much the appearance which a bath sponge would present in 

 section, fraying out, as it were, into the surrounding fluid ; and 

 one may fairly conclude that during life it possesses the power 

 of sending out processes into the fluid surrounding the embryo, 

 and that the superficial vacuoles open to the exterior. In 

 short, I am inclined to think that this surface ectoderm during 

 Stages E to F has a nutritive function, absorbing the fluid in 

 which the embryo lies, and it seems to me conceivable that the 

 placenta described by Kennel in the Trinidad species may be 

 a more specialised organ of the same nature. During the pro- 

 gress of Stage F the nuclei which have hitherto been placed in 

 the deep parts of the layer (PI. XXXV, figs. 23 a — e, 25) acquire 

 a superficial position, excepting in the hump, where they retain 

 their deep position until after Stage g. Contemporaneously 

 with this change the deeper parts of the ectoderm become 

 filled with very large vacuoles, so that the protoplasm is re- 

 duced to fine cords, passing inwards from the superficial nucleated 

 layer. This vacuolated deeper part of the dorsal ectoderm 

 now becomes much reduced, so that in Stage f the dorsal 

 ectoderm consists mainly, if not entirely, of a thin layer of 



