960 SPECIAL PHYSIOLOGY. 



embryonal vascular portion of these membranes, whether it be a diffused, 

 cotyledonous, zonular, or discoid placenta, is always detached. In the 

 case of the zonular and discoid forms of placenta, where a true decidua 

 is developed, a part of the maternal tissues is also separated at the 

 same time. Where there is no decidua, as in the diffuse and cotyle- 

 donous forms, the foetal villi are merely detached from the surfaces or 

 recesses into which they fit. In the latter cases, parts of the maternal 

 tissues, especially of the veins and venous lacunae, come away. Hem- 

 orrhage is ordinarily quickly arrested, owing to the obliquity of the 

 passages leading into the deeper uterine veins, and to the firm contrac- 

 tion of the uterine walls. If these become relaxed, arterial, but not 

 venous, hemorrhage may occur. 



DEVELOPMENT OF THE ORGANS. 



The share taken by each of the three germinal layers of the blasto- 

 derm in the formation of the several systems of organs having been 

 described, their special development may now be considered. 



The Skeleton, Muscles, and Integuments of the Body. 



The vertebral column is developed from the vertebral plates of the 

 middle germinal layer, found on each side of the vertebral groove and 

 chorda dorsalis. As the vertebral groove closes along the back of the 

 embryo, first opposite. the cervical and dorsal regions, small square 

 masses are seen on each side of the median line, in the inner thicker 

 portion of the vertebral plates. These were formerly considered to be 

 the rudiments of the bodies of the vertebrae only, and were named the 

 primitive or primordial vertebrce ; but they are better named the dorsal 

 segments, for other structures, besides the vertebrae, are developed from 

 them. In the thoracic region, the posterior ends of the ribs, and, 

 throughout the whole length of the vertebral column, the roots of the 

 spinal nerves, together with the ganglia on the posterior roots, which 

 are of great proportional size, the spinal muscles and the cutis cover- 

 ing these parts, are thus formed ; a small portion only, therefore, is 

 developed into the future vertebrae. The part which forms the skin 

 and spinal muscles is named the dorsal plate or division, whilst the 

 rest is called the ventral plate or division. The innermost portion of 

 this latter, which is called the vertebral plate, grows inwards, and sur- 

 rounds the chorda dorsalis, so as to inclose it in a thick, continuous, 

 membranous sheath ; this divides anew, transversely, into annular 

 portions, corresponding with the bodies of the future cervical, dorsal, 

 lumbar, sacral, and coccygeal vertebrae, into which they are developed, 

 by passing first into a cartilaginous, and then into an osseous, state. 

 The body of the atlas joins the axis, to form its odontoid process. The 

 remains of the chorda dorsalis are traceable, for a long time, through 

 the centre of the bodies of the vertebrae, but ultimately they become 

 absorbed. In the lowest or Myxinoid Fishes, however, the chorda 

 dorsalis is recognizable throughout life. Between the vertebrae thus 

 formed, an intermediate soft tissue becomes developed into the interver- 



