in the Fcetus of Vertebrated Animals. 801 



sion, and, by a peculiar change, is converted into the vascular 

 and mucous layers (Fig. 9. d d'). 



Towards the 25th hour, when the layers of the germinal 

 membrane cover nearly a third of the circumference of the yolk, 

 they no longer retain their flat and uniform position, but begin 

 to exhibit various folds, which afterwards serve for the forma- 

 tion of the cavities of the body. That part of the germinal 

 membrane which lies immediately before the cephalic extre- 

 mity of the embryo is bent down into a fold, so as to make a 

 depression on the surface of the yolk ; and some time after- 

 wards, a similar fold is formed behind the caudal extremity. 

 As these folds of the germinal membrane increase, they gra- 

 dually turn in below the foetus at its head and tail, and their 

 margins {ef, Figs. 8, 10, 12, 9, 11, 13), approach one another 

 under the abdomen, which at this period always lies next the 

 substance of the yolk. As the layers of the germinal mem- 

 brane are bent down in a similar manner towards the sides also 

 of the spinal canal (Fig. 13. g), there is formed under each end 

 of the embryo a shut sac or cavity, which communicates with 

 the yolk by an opening, common to both, left in the middle 

 (Fig. 13. ef). The two shut sacs thus formed indicate the 

 rudimentary state of the intestinal tube: the anterior corre- 

 sponds to the oesophageal portion of the intestine, the posterior 

 to the lower part of the large intestine. 



The first rudiments of the heart appear, towards the 27th 

 hour, on the lower side of the oesophageal canal (Figs. 12, 13, /<), 

 at the place where the layers of the germinal membrane are re- 

 flected from the edge of the anterior shut sac which they form 

 in the embryo. In forming this fold, the mucous layer (Fig. 

 13, e), is reflected farthest inwards, the serous layer advances 

 least, and the space between them, occupied by the vascular 

 layer, is filled up by a dilated part of this layer, the rudiment 

 of the heart (Ji). 



About the same time that the development of these rudimen- 

 tary parts of the embryo takes place, the surrounding disk of 

 the cicatricula is also considerably changed. The whole cica- 

 tricula continues to expand, and to cover more of the surface 

 of the yolk. That part of the mucous and vascular layers which 

 surrounds the transparent area, becomes thicker and more spongy 



