DEVELOPMENT OF THE EMBKYO. 745 



will be found to have become more prominent where it passes over the 

 germinal space (fig. 372, i, D). The outer layer of the blastoderm (c) 

 has become thickened at e into the first rudiment of the dorsal portion 

 of the embryo ; but the mucous layer (A) and the vascular layer (B) 

 have as yet undergone little alteration. 



(2115.) At the commencement of the second day (fig. 372, 2), the 

 anterior portion of the embryo is dilated, and bent down so as to inflect 

 the three membranes of the blastoderm at this point. 



(2116.) At the conclusion of the second day this inflection is carried 

 still further, and from the vascular layer a single pulsating cavity 

 (fig. 372, 3, h), the punctum saliens (the first appearance of a heart), has 

 become developed ; so that considerable advance is already made towards 

 that disposition of the foetus and its membranous investments repre- 

 sented in the next figure, to which we now beg the reader's attention. 



(2117.) The serous membrane (fig. 372, c) has at the third day become 

 reflected to a considerable distance over the back of the foetus ; at one 

 extremity investing the head with a serous covering, while at the oppo- 

 site it in like manner covers the tail : it is this reflection of the serous 

 layer which forms the amnion, as will be observed in fig. 373, where 

 the amniotic sac (c) is completed. 



Fig. 373. 



Embryo in a more advanced stage. 



(2118.) The mucous layer (A) is now seen to line the as yet open 

 space which is to form the abdominal cavity, and by its inflections 

 gives birth to the rudiments of the abdominal viscera. 



(2119.) From the vascular layer (B) has been developed the heart, 

 now composed of two chambers (a, >), and the branchial arteries (c), 

 which join to form the aorta (m), exactly as in the Menopoma (fig. 343). 

 The allantois (p), the uses of which will be described hereafter, like- 

 wise begins to make its appearance*. 



(2120.) At the fifth day (fig. 374) the lineaments of the viscera 

 become tolerably distinct. The sac of the amnion (c) is completed ; the 

 liver (i) and the lungs (e) begin to show themselves ; and the bag of 

 the allantois (p) is largely developed ; still, however, the heart (a, b) is 

 that of a fish, and the aorta (m) formed by the union of the branchial 



* Des Branchies et des Vaisseaux branchiaux dans les Embryons des Animaux 

 vortebres, par Prof. Ch. Ernst r. Baer (Ann. des Sci. Nat. torn. xv.). 



