54 THE CHANGES FROM STAGE A TO F. 



(Plate YI, fig. 6 c), and that which is placed on each side 

 of the anusj becomes reduced to an extremely thin layer 

 (Plate VI, fig, 6 c, d), the character of which will be obvious 

 from an inspection of the figures. In Stages d and e, the ventral 

 ectoderm retains the characters already described, but its width 

 becomes less, the lateral thickenings having somewhat ap- 

 proached one another on the ventral surface (figs. 9, 25). The 

 dorsal ectoderm, on the other hand, becomes in Stage d some- 

 what reduced in thickness (Plate VI, figs. 9 — 13), though it 

 never becomes as thin as the ventral portion. Subsequently, in 

 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 (PL VII, fig. 25), where indeed it lasts longer 

 and is more marked than elsewhere, and gives rise to the pro- 

 minence marked d in the figures of Plate II. 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 difl'erent specimens, and no doubt depends 

 to a certain extent on the anaount 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 enabryo, 

 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- 



