CHANGES IN THE OVUM. loi 



a part in the nutrition and haematosis of the fcEtus and young animal. It 

 may be that, like the spleen, it assists in converting the white corpuscles 

 of the blood into red corpuscles. 



The Digestive Apparatus. 



The development of the digestive apparatus comprises the formation 

 of the alimentary canal and the organs attached thereto. The alimentary 

 canal begins to appear after the first outlines of the nervous centres and 

 the vascular apparatus have been manifested. We have already described 

 the manner in which the intestinal cavity was formed from the inner lamina 

 of the blastoderm. This cavity, for convenience of description, may be 

 divided into three portions ; the anterior iyitestine, which originates the 

 pharynx and oesophagus ; the middle intestine, which becomes the stomach 

 and intestines proper; and the posterior intestine, which constitutes the 

 rectum. The chief, or middle intestine, is at first a cylindrical uniform 

 tube, the diameter of which is afterwards modified to constitute the organs 

 comprised between the oesophagus and rectum. 



The mouth begins by a depression or cul-de-sac, which is limited by the 

 maxillar}'- tubercles ; it increases as it dips towards the pharj'-nx, from 

 which it is only separated at last by a thin membrane ; this is eventually 

 absorbed, and the mouth then communicates with the commencement of 

 the digestive canal. Towards the third month the mouth is confounded 

 with the nasal fossae, but after this the palatine bones appear, and finally 

 isolate the two cavities. 



The to7igue is at first only a small protuberance from the maxillary 

 tubercles, but is completed by the addition of a little growth from the 

 second branchial arch. Its ephithelium and glands are derived from the 

 external layer of the blastoderm ; they are apparent at the third or fourth 

 month. 



The pharynx and oesophagus lengthen and widen as the foetus grows ; the 

 latter at first communicates with the trachea, but it gradually closes, and 

 ends by separating entirely from that tube. 



The stomach is formed by the dilatation of the anterior part of the mid- 

 dle intestine. This dilatation is fusiform, its larger axis being longitudi- 

 nal ; but it soon curves on itself, and then this axis becomes transversal. 



In ruminants the stomach is single when it first appears, though it is 

 not long before furrows are observed on its outer surface ; while inter- 

 nally the particular septa are seen as in adult life. In the foetus the 

 stomach is small, but its volume increases rapidly after birth, when the 

 animal commences to take solid food. During the sucking period in 

 ruminants, there is a predominance of the fourth over the other gastric 

 compartments; but as soon as the young animal begins to consume 

 fibrous aliment, the rumen increases rapidly, until it is by far the largest 

 cavity. 



The intestines are at first of uniform calibre, though in a short time it is 

 easy to distinguish the different sections of which they are ultimately com- 

 posed. In hoofed animals the caecum appears very early ; it is situated 

 near the omphalo-meseraic duct, which is detached from the extremity of 

 an intestinal loop that is drawn towards the umbilical ring ; while the 

 latter is becoming obliterated, this loop ascends in the abdominal cavity. 

 The intestines are quite smooth on the inner surface during the first, two 

 months, and towards the third month show the villi and glands of Lieb- 

 erktihn ; the Brunnerian glands and the follicles are only seen later. 



