510 URO-GENITAL SYSTEM. 



vesicle is not divided (middle portion of the brain) and 

 forms the region of the corpora quadrigemina with the 

 aqueduct of Sylvius (continuation of the medullary canal); 

 3. The posterior or third cerebral vesicle divides, like the first, 

 into two portions; one of which, that nearest to the middle 

 part of the brain, will form the protuberance, or medulla 

 oblongata, and the cerebellum (posterior part of the brain) ; 

 the other of these divisions, a direct continuation of the 

 spinal cord, will form the rachidian bulb (medulla oblongata, 

 strictly speaking) ; this is the point in which the medullary 

 or neural canal does not completely close, but persists in its 

 original form of a groove and constitutes the floor of the 

 fourth ventricle. 



The peripheral nerves are formed in their proper place at 

 the expense of the middle layer (mesoblast) of the blastoderm. 

 The optic nerve and retina form an exception and are repre- 

 sented by a diverticulum of the encephalic substance (see p. 

 425, Fig. 113.) 



The ganglia of the great sympathetic are also formed in 

 their proper places, independently of the ccrebro-spinal sub- 

 stance, and from the middle layer (mesoblast) of the blasto- 

 derm, as \ve have already learned, in treating of the semi- 

 lunar ganglia of the abdominal portion of the sympathetic 

 system (see p. 277). 



b. Circulation in the Embryo. The circulation of the 

 embryo depends upon its method of nutrition. As we have 

 already learned, this nutrition of the embryo may be effected 

 in three different ways : First, by the simple and direct assim- 

 ilation of the albuminous substance in which the ovum is 

 immersed ; no system of circulation is required for this 

 simple form of imbibition. Second, by an assimilation of the 

 contents of the umbilical vesicle ; these contents are conveyed 

 to the embryo by a peculiar system which forms the primary 

 or omphalo-mesenteric circulation (sometimes written om- 

 phalo-mesaraic). Third, by an interchange with the mater- 

 nal blood through the placenta ; this method of nutrition is 

 fulfilled by the secondary or placental circulation. 



1. The system for the primary circulation commences 

 with the formation of the heart ; this organ is at first repre- 

 sented by a cylinder of embryonic globules ; soon the sur- 

 rounding globules become organized into muscular fibres, 

 whilst those at the centre undergo a partial dissolution and 

 form the first blood. Simultaneously the heart, which at first 



