326 



THE CAT. 



[CHAP. x. 



and lungs grow out from that canal vent-rally. The renal and sexual 

 organs arise close to the bifurcation of the ventral laminae into the 

 somatopleures and splanchnopleures, and the limbs bud forth as rounded 

 processes, the distal ends of which subsequently grow out into digits, 

 while the limbs themselves become flexed in reverse directions. On 



each side of the body, close behind 

 the head, certain apertures ap- 

 pear, which lead from the exterior 

 to what becomes the pharynx. 

 These openings are the visceral 

 clefts, and their interspaces are 

 the visceral arches. Almost all 

 the clefts disappear before birth. 

 The formation of these various 

 organs will be detailed subse- 

 quently, but their relations to the 

 three primary layers, from which 

 the whole of them are built up, 

 may be summarized as follows. 

 The epiblast gives rise to the 

 epidermis of the skin, the ner- 

 vous centres, and the organs of 

 sense. The hypoblast forms the 

 epithelium of the alimentary 

 canal, except its two ends, and 

 of the glands which open into 

 it. The mesoblast forms the in- 

 ternal skeleton, the muscles, connective tissue, peritoneum and 

 pleurae, and the vascular and secreting organs generally. It forms, 

 therefore, the great bulk of the cat's body. 



6. During the whole process of development the germ is 

 nourished by absorption. Within the Graafian follicle it profits by 

 the cells of the discus proligerus and membrana granulosa, and when 

 cast forth from its follicle into the cavity of the uterus it absorbs 

 nutriment from the secretions of the uterine walls by processes, or 

 villi, which grow forth on all sides from the surface of its chorion. 

 It also feeds upon the contents of the umbilical vesicle, absorbing 

 nutriment thence by the help of the vessels which there circulate, 

 and which are at first of great relative size and importance. With 

 the development of the allantois, however, a new condition obtains. 

 That organ is destined to convey out embryonic blood-vessels to the 

 surface of the ovum, so that they may there be placed in intimate 

 relation with the blood-vessels of a special, corresponding maternal 

 structure, which is formed in the wall of the uterus around the 

 circumference of the therein-contained ovum. 



While the ovum is undergoing the incipient stages of develop- 

 ment, corresponding changes take place in the maternal structures. 

 The presence of the impregnated ovum within the uterus is ac- 

 companied by the growth, on the inner surface of that organ, of a 



Fig. 149. SECTION THROUGH COUNU OF UTERUS, 



SHOWING THE TWO POINTS OF ATTACHMENT 

 OF THE ZONARY PLACENTA, THE EMBRYO 

 BEING REMOVED. 



pi. Placenta. 



>/. Its maternal portion sending processes 



between. 

 t. Tufts projecting from chorion or membrane, 



enveloping embryo. 

 vc. The umbilical cord. 

 am. Amnion. 

 ut, ut. Walls of the uterus. 



