CONCLUSIONS. 107 



XV. CONCLUSIONS. 



Based on the observations recorded in the foregoing sections, the following 

 conclusions seem warranted: 



(1) During the early part of the growth of the pig embryo there is no extra- 

 ventricular spread of the cerebro-spinal fluid. The first extension of the ventricular 

 fluid into the periaxial tissues occurs in pig embryos of 14 mm.; the adult relation- 

 ship of the ventricular and meningeal cerebro-spinal fluid is established in pig 

 embryos of about 26 mm. 



(2) The ventricular cerebro-spinal fluid escapes into the periaxial tissues 

 through two areas of ependymal differentiation in the roof of the fourth ventricle. 

 Both of these areas differentiate at a slightly earlier period than that at which they 

 function actively. The area membranacea superior undergoes a gradual regression 

 and obliteration due to the changing form of the rhombic roof; the area mem- 

 branacea inferior gradually occupies the major portion of the velum chorioidea 

 inferior. 



(3) The embryonic cerebro-spinal fluid, as evidenced by the replacement with 

 a true solution, spreads from the ventricles into the mesenchymal tissue about the 

 central nervous system. It does not penetrate the cranial or vertebral blastemal 

 condensations, nor does it invade the pial cellular layer. 



(4) The subarachnoid spaces arise by a process of breaking-down of the peri- 

 medullary mesenchymal syncytium and a dilatation of the existent mesenchymal 

 spaces. This phenomenon of the enlargement of the mesenchymal spaces is asso- 

 ciated with the presence in the spaces of an increased amount of albumen. The 

 process occurs at a period slightly later than that at which the initial flow of the 

 cerebro-spinal fluid into the spaces is recorded. 



(5) The dura mater, arachnoid, and pia mater develop out of the perimedullary 

 mesenchyme. The arachnoid trabeculae are left by the breaking-down of the original 

 mesenchymal strands, while the outer arachnoid membrane is formed, together with 

 the inner surface of the dura, by a separate mesenchymal condensation. The dura 

 develops between this secondary line of condensation and the embryonic skull. 



(6) There is indicated a very close relationship between the tufting of the 

 chorioid plexuses of the fourth ventricle and the first extraventricular spread of the 

 cerebro-spinal fluid. 



(7) By means of the method of replacement it is possible to demonstrate peri- 

 neural spaces as far out along the nerve trunks as the peripheral ganglia. The 

 extensive injections of the perineural spaces along the segmental nerves are not 

 obtained by the method of replacement. 



The work, of which this paper forms the report, was done in the Anatomical 

 Laboratory of the Johns Hopkins Medical School. It was largely due to aid 

 received from the Department of Embryology of the Carnegie Institution of Wash- 

 ington that the completion and scope of this paper were possible. The writer 

 gladly acknowledges his indebtedness to the Carnegie Institution. 



January, 1916. 



