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CHAt. X.J !rilE DEVELOPMENT OF THE CAT. 345 



The PERITONEUM, SO complicated in the adult, arises as simply' the 

 lining membrane of the adjacent surfaces of the two diverging plates 

 (somatopleure and splanchnopleure), into which each ventral lamina 

 splits. Therefore, for a time, the primitive peritoneal ca\'ities of the 

 two sides are continuous with each other, not on the ventral side of 

 the embryo only, but, as before pointed out, over its back, between 

 those doubled folds of somatopleure which ascend on all sides to form 

 the amnion (Fig. 148, -"■'). 



As the somatopleures descend and gradually embrace the ali- 

 mentary canal — formed by the splanchnopleures — they enclose 

 more and more the peritoneal cavity, and when they come to meet 

 together below, they do so completely (Fig. 147, C, D). 



It has already been seen that the divergence of the middle part 

 of the alimentary tube from the vertebral column produces an in- 

 cipient mesentery. By a continuation of this process the alimentary 

 canal comes to be slung, from the skeletal axis, in a mesenteric 

 peritoneal fold. With the increasing complexity of contortion into 

 which the alimentary canal is thrown by its great increase in length, 

 a corresponding increase of complexity is induced in the mesenteric 

 folds by which it is slung. A pouch of it, having of course four 

 walls, becomes protruded ventrally between the stomach and trans- 

 verse colon, and so forms the omentum. 



§ 14. The BLOOD makes its first appearance in the interior of the 

 primitive blood-vessels, as already described. Both blood and 

 blood-vessels begin to appear first in that layer of the blastoderm 

 which is immediately adjacent to the embryo. 



Certain of the primitive mesoblastic cells of the embryo become 

 masses of corpuscles in fluid, enveloped by more solid nucleated pro- 

 toplasm — the primitive walls of the vessels. These arise, as before 

 explained,* from the junction of radiating processes of adjacent cells, 

 the cavities of which open one into another. Blood-vessels are at 

 first confined in the embryo's body to the splanchnopleure. 



The HEART arises as an elongated cavity in the splanchnopleure 

 of the anterior end of the embryo. It consists at first merely of 

 nucleated cells, and its rythmical contractions begin while in this 

 incipient, merely cellular condition, and before the muscular fibres, 

 which subsequently arise, are developed. Its first form is that of 

 an elongated vesicle, or tube, into the posterior end of which two 

 veins open, while a large artery proceeds from its front end. It 

 increases in size and thickness, and bends over in a crescentic loop, 

 projecting downwards and towards the right. At the same time two 

 sHght constrictions divide it more or less into three successive 

 portions. The hindermost of these (that into which the two veins 

 open) becomes the future auricles. The next portion becomes the 

 ventricles, while the most anterior segment, from which the great 

 artery proceeds, is called the hidbus arteriosus. As the heart be- 

 comes more and more beet, the auricular portion approaches the 



* P. 331. 



