GENERAL HISTOLOGICAL DIFFERENTIATION OF CEREBRO-SPINAL SPACES. 65 



relatively of great importance when compared to the possible influence of the pres- 

 ence and circulation of cerebro-spinal fluid on this periaxial tissue. This seems to 

 be the most important factor, an internally-modifying influence to which the peri- 

 axial mesenchyme is subjected in the formation of an arachnoid and its subarach- 

 noid spaces. It will therefore be from this standpoint that the development of the 

 spaces will be discussed; for, as has already been pointed out, the periaxial mes- 

 enchyme becomes a functionally active tissue for the circulation of the cerebro- 

 spinal fluid at a stage when differentiation has not begun. On this basis, the 

 lack of differentiation shown in the periaxial mesenchyme in the stages before 

 the ventricular cerebro-spinal fluid is poured into the mesenchyme in the neighbor- 

 hood of the roof of the fourth ventricle is not surprising. The character of the 

 periaxial mesenchyme in the early stages is reproduced in numerous photomicro- 

 graphs (figs. 25, 49, 51, and 53). The mesenchyme is here characterized by a rather 

 dense meshwork of cytoplasmic processes, interspersed by a considerable number of 

 oval nuclei. The content of the interstices in albumen, as judged by the persisting 

 coagula, is very small. This picture of the periaxial mesenchyme persists until 

 cerebro-spinal fluid is poured from the ventricle through the area membranacea 

 superior. 



As will be seen in figure 3, the first indication of an extraventricular spread of 

 the replaced fluid in the ventricles occurred in a pig embryo of 14 mm. At this 

 stage the membranous area in the superior portion of the roof of the fourth ventricle 

 has already become well differentiated. The fluid from the ventricles, however, 

 does not reach any considerable spread until after a length of 18 mm. is attained; the 

 periaxial spread during this period of growth is wholly confined to the peribulbar 

 tissues. It is quite important in this connection that the first obvious differentia- 

 tion of the mesenchyme for the formation of the arachnoid should appear during 

 this period and should involve the peribulbar tissues. 



The first change to be noted in the transformation of primitive mesenchyme 

 into the future arachnoid is an obvious thinning of the structure with a decrease 

 in the number of nuclei per unit-volume. This is made out in a photomicrograph 

 (fig. 57) of a section from a human embryo 14 mm.* long, when contrasted with a 

 similar mesenchymal area posterior to the ventricular roof (fig. 53). In the pig 

 embryo this thinning of the mesenchyme is as obvious at this early stage. 



The process of dilatation of the mesenchymal spaces at this stage hardly seems 

 to concern a direct disruption of the syncytial strands, but resembles more the spread- 

 ing of the cell-bodies by the introduction of more fluid into the tissue spaces. This 

 process would certainly result in an appearance similar in every way to that repre- 

 sented by figures 35 and 57. It probably also concerns other factors, as, possibly, 

 the growth of the whole embryo without a corresponding degree of mesenchymal 

 proliferation. 



In a human embryo of 17 mm. (No. 576) evidences are apparent of such a 

 thinning of the mesenchyme about the medulla. Thus, in figures 58 and 59, from 



*This embryo measured 14 mm. on the slide. 



