THE DEVELOPMENT OF THE EMBRYO 255 



ject; it will, therefore, be necessary to condense the description as far as 

 possible. 



Development of the Nervous System. — In the early embryo the 

 formation of the line of cells below the primitive trace was described as the 

 chorda dorsalis, the basis of the future backbone or vertebral column. 

 Conversion of the gelatinous mass of cells into bone is the simple result 

 of the deposition of bone-earth, calcium phosphate and carbonate mainly. 

 With the ossification is associated the necessary elaboration of form of the 

 bones, ending in the development of the bodies, arches, and processes of the 

 vertebral bones, which are divisible into neck, back, loins, and tail — i.e. 

 cervical, dorsal, lumbar, sacral, and coccygeal bones. At the anterior 

 part of the vertebral column a prolongation of the structures occurs, to form 

 the cranium to contain the brain, and next the bones of the face are formed 

 from a series of arches derived from the visceral laminse, which have been 

 described. 



The four limbs or extremities at the same time are growing from the 

 laminae which form the boundaries of the trunk, and it is noticeable that 

 in all vertebrate animals the four extremities are at first identical in 

 form, whether their ultimate use is to be for walking, grasping, swimming, 

 or Hying; in other words, whether the extremities are to be finally feet or 

 hands, or fins or wings, they all have the same shape at first. The highest 

 mammal in the course of embryonic development exhibits some of the 

 features of the reptile, fish, and bird, a good example of evolution in a com- 

 pressed form never exciting any astonishment, because it is never seen by 

 the ordinary eye, being hidden in the membi'anes which invest the ovum, 

 and only to be detected by elaborate and minute dissection by a practised 

 anatomist skilled in the use of the most delicate instruments. 



Development of the Heart and Vessels.— In its primitive con- 

 dition the heart is a mass of cells to which, as already described in the 

 embryo, the vessels of the vascular area tend and ultimately reach, forming 

 the rudiments of the circulatory system. 



Cavities are constructed in the mass of cells representing the heart, 

 which become separated to form the ventricles and auricles. Blood- 

 vessels which were formerly only red lines acquire size and shape, and 

 divide themselves into arteries and veins, and gradually the complicated 

 mechanism which is described in the section on the anatomy of the organs 

 of circulation is elaborated from a few clusters of cells. 



Long before birth the foetus possesses a perfectly complete set of organs 

 connected with the circulation, difi"ering in a few details of construction 

 to meet the peculiarities of the ftiatal environment. To understand the 

 circulation of the blood in the unborn foal, it is desirable to refer to 



