io8 ANIMAL BIOLOGY. [Parti. 



The Embryonic Appendages and Foetal Membranes. We have 

 now to consider certain important structures which characterise 

 the fowl and the rabbit, but which are absent in the frog. The 

 first of these is the amnion. 



Eevert for a moment to our rough and ready illustration, 

 with knife and handkerchief, of the manner in which the 

 embryo is folded off. Suppose that all around our mimic 

 embryo we fold the handkerchief in a ridge, and then, thread- 

 ing a piece of cotton along the summit of this ridge, draw the 

 ends of the string together, and so pull the folds over the 

 embryo until they meet above it. This again illustrates, in 

 a rough but comprehensible fashion, the way in which, in 

 the chick embryo, the amnion folds grow up all round, and 

 eventually meet and coalesce above the embryo. This illustra- 

 tion, however, fails to mark an important distinction. It shows, 

 indeed, that in the folding off of the embryo the folds are 

 inwards, while in the formation of the amnion they are out- 

 wards. But it does not show that in the former process the 

 folds affect the whole thickness of the blastoderm, while in the 

 latter, the formation of the amnion, only the somatic layer is 

 affected. This will, however, be readily seen in Fig. 38, ii. and 

 iii., which are diagrammatic cross-sections of the embryo. In ii., 

 in which the folding off of the embryo is omitted for the sake 

 of clearness, the amnion fold (am, /.), is seen at the edge of the 

 somatic layer. In iii., in which the embryo is represented as 

 folded off, the amnion folds have nearly coalesced above the 

 embryo. The folds are hollow, and enclose a space (5. s. c.) 

 which is continuous with the body-cavity . (b. c.). It must be 

 remembered that such folds extend all round the embryo, the 

 head-fold being very well marked. The edge of its outer limb 

 is shown in Fig. 37, i., ii., iii., the inner layer being so closely 

 applied to the head as not to be distinguished. At last all 

 these folds unite and coalesce above the embryo ; the outer 

 layers unite and become continuous ; the inner layers do the 

 same ; and thus the space (5. s. c., Fig. 38, iii. and iv.) between 

 them becomes continuous. The inner layer is known as the 

 true amnion, the outer layer as the false amnion, The false 



