66 DEVELOPMENT OP CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



this specimen, the cellular decrease can be made out both in the region of the roof 

 of the fourth ventricle and around the basilar surface of the medulla. It will be 

 noted that the differentiation (i. e., the thinning) about the roof has proceeded more 

 rapidly than along the anterior bulbar surface. This is perhaps to be expected in 

 view of the initial pouring-out of the cerebro-spinal fluid into the mesenchyme just 

 posterior to the roof. 



In this mesenchymal differentiation a slightly increased amount of albuminous 

 coagulum may be noticed. The truth of this is made obvious by an examination 

 of figure 61, a photomicrograph from a human embryo of 17 mm. The almost 

 entire freedom of the mesenchyme from albuminous detritus is most noticeable at 

 earlier stages. 



As was pointed out in the description of the results of replacing the cerebro- 

 spinal fluid, a marked change in the rate of development of the cerebro-spinal spaces 

 in the pig-embryo ensues just after attaining the length of 18 mm. Within the 

 growth of 2 mm. the injection spreads completely down the spinal cord and about 

 the basilar structures of the cerebral cavity. This rapid extension finds its analo- 

 gous process in the equally rapid changes which may be traced in the periaxial 

 mesenchyme. Thus, in figure 72, a photomicrograph from a sagittal section of a 

 pig embryo of 18 mm., the whole nervous tissue appears surrounded by a very thin, 

 lightly staining tissue; this is the periaxial mesenchyme, which is undergoing its 

 rapid metamorphosis. It will be noticed in this figure that the posterior structures 

 (rhombencephalon) are surrounded by a much less dense mesenchyme than are 

 the anterior (mesencephalon) . This relative differentiation between the bulbar 

 tissue and that around the mid-brain is only of temporal character; the mesenchyme 

 about the medulla, as has already been pointed out, begins to differentiate first, 

 the differentiation of the mesenchyme about the other nervous structures following 

 somewhat later. 



Figure 73 is a photomicrograph of higher power, taken from the squared area 

 in figure 72. It shows to what a surprising degree the mesenchymal differentia- 

 tion has proceeded during a few millimeters' growth. Two striking features of the 

 process are brought out in this reproduction. In the first place, many of the mesen- 

 chymal trabeculse have apparently been broken down, sacrificed to a few larger 

 remaining strands. The cells connected with the destroyed trabeculse appear to 

 recede until one of the heavier surviving strands is met with, when they adhere and 

 apparently aid in the future development of a permanent arachnoid trabecula. The 

 second feature of importance in figure 73 concerns the large amount of albumen seen 

 in the periaxial space. There is here a much greater amount of albumen than is ever 

 found in the periaxial mesenchyme before the differentiating process which results 

 in the future subarachnoid space has become definite. The occurrence of this large 

 amount of albuminous coagulum is apparently related directly to the outflow of the 

 embryonic cerebro-spinal fluid, for the embryonic fluid is very rich in protein 

 material, as can be readily seen by the partial filling of the embryonic cerebral 

 ventricles with the clotted albumen. 



