CHANGES IN THE OVUM. 89 



SECTION III. DEVELOPMENT OF THE FCETUS. 



Having studied the conception and partial development of the young 

 ■creature, and described the envelopes which surround it, we will proceed 

 to notice the various changes which occur in it until gestation is com- 

 pleted and parturition is about to take place. 



This division of our subject is of much importance in several respects, 

 but more especially with regard to the relation it bears to teratology — the 

 branch of science which treats of congenital malformations and mon- 

 strosities. 



The transition from the condition of the embryo, when the young an- 

 imal has scarcely assumed a definite form, to that of the foetus, when it 

 presents the lineaments of the species to which it belongs, is very 

 gradual. 



The dorsal cord, as has been stated, is a cylindrical body developed 

 above the primitive furrow, with slightly attenuated extremities, and at 

 each side small opaque quadrangular masses, the vertebral lamincE, which 

 are in reality the protovertebrce, or first rudiments of the vertebrae. Each 

 of these masses is perforated by a small opening, and is resolved into 

 three portions : theprotovertebral cavity, the miisctilar la7nma,si\M2i\.ed2ihovQ 

 the cavity, and the protovertebra placed below the cavity. The muscular 

 lamince, increasing in volume, are inflected upwards and at last unite on 

 the median line of the back, chiefly forming the muscles of the vertebral 

 furrows ; they also send off prolongations downwards, which concur in 

 the development of the intercostal and abdominal muscles, as well as 

 those of the limbs. The protovertebrce bend upwards and downwards on 

 each side, so as to enclose the protovertebral cavity or spinal canal, and 

 the dorsal cord ; the upper ring represents the rudiments of the annular 

 portion of the vertebrae ; while the lower ring and the dorsal cord consti- 

 tute the vertebral bodies and the discs uniting them. 



The lateral la7}iince arise from the portion of the middle layer of the 

 blastoderm placed on each side of the vertebral laminae. In the region 

 of the trunk, these laminae are separated for a certain time from the lat- 

 ter ; but in the cephalic region they are always adherent to them, and at 

 this part they are usually designated the cephalic lamince. The proper lat- 

 eral lamiiKB are divisible into two layers, external and internal, united by 

 a.middle layer; they comprise between them a space which becomes the 

 pleuro-peritoneal cavity, after the formation of which the lateral are joined 

 to the vertebral lamina. The internal or Jibro-intestinal layer envelops 

 the deeper portion of the blastodermic layer or intestinal furrow, the um- 

 bilical vesicle, and the allantois ; it constitutes the fibrous and vascular 

 parts of these membranes, and carries the vessels to the inner face of the 

 chorion. The external or cutaneous layer is developed in two ways : above, 

 it glides between the muscular laminae and the foetal portion of the exter- 

 nal layer of the blastoderm to form the cutaneous envelope on the back j 

 below, it separates into two leaves, which receive between them the pro- 

 longations of the muscular laminae destined to constitute the intercostal, 

 abdominal, and other muscles of this part of the body. Of these two 

 secondary leaves, the external forms the skin of the trunk, and the inter- 

 nal the parietal layer of the peritoneum. The cutaneous laminae also fur- 

 nish an extra-fcetal prolongation — the fibrous layer of the amnion. 



The middle or tnesenteric lanwice join at the median line, and in their 

 substance are developed the Wolffian bodies, or antecedent deciduous 

 kidneys, and the principal vessels of the trunk. 



