44 DEVELOPMENT OF CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



THE AREA MEMBRANACEA INFERIOR IN THE PIG EMBRYO. 



The inferior portion of the fourth ventricle shows no evidence of a differentiation 

 from the typical lining ependyma until the length of 15 mm. is reached. In this 

 development consideration must be given to the factors concerned in the process. 

 It will be recalled that in the younger embryos, both pig and human, up to and 

 including a length of 9 mm. the whole roof of the ventricle is occupied by the epi- 

 thelial-like cells. With rapid growth of the medulla and corresponding enlargement 

 of the fourth ventricle the roof becomes elongated and widened. This process 

 results in the isolation of the area composed originally of the epithelial-like cells and 

 the subsequent formation of the superior membranous area. The epithelial-like 

 cells remain in the superior portion of the enlarged ventricular roof, while the whole 

 inferior half is composed of the densely staining, typical ependyma. The division 

 of the roof by the laterally developing chorioid plexuses becomes evident in pig 

 embryos of 14 mm. At this stage the whole inferior portion shows a ventricular 

 lining composed of the typical ependyma. 



The first indication of a differentiation in this inferior half of the roof was found 

 in a pig embryo of 15 mm. This is illustrated in figures 70 and 71. The sagittal 

 section from which these photomicrographs were taken is near the mid-line of the 

 embryo, as is indicated by the partial section of the central canal of the spinal cord 

 (fig. 70). The division of the ventricular roof into two parts is also indicated in 

 figure 70 by the imagination of the chorioid plexus. The squared area in the lower 

 half is reproduced in figure 71 under higher magnification; here the first evidence of 

 an ependymal differentiation is observed. The dense line of the typical ependyma 

 appears from both sides, but in the center of this ventricular lining a small area of 

 differentiation is seen. This area, isolated by the abruptly terminating pyknotic 

 ependymal elements, is composed of two or three layers of less deeply staining cells. 

 The nuclei are round, rather larger than those of the adjacent mesenchyme, and 

 contain little chromatin. The cytoplasm stains fairly well with eosin and is not 

 scanty in amount. The cells resemble those epithelial-like elements which so 

 largely make up the ventricular roof in the earlier stages. No albumen is found 

 near this point of differentiation, although the whole ventricular cavity is filled with 

 the normal amount. In figure 70 the marked zone of the area membranacea superior 

 may easily be seen. 



After this initial indication of a differentiation in pig embryos, the further dif- 

 ferentiation of the tissue proceeds but slowly until the length of 18 mm. is attained. 

 Thus, in a similar specimen from an embryo pig of 18 mm. the area of differentiation 

 is not greatly increased in size. This is shown in figures 72 and 73. In the higher- 

 power figure (fig. 73) both the superior and inferior membranous areas can be made 

 out by the attachment to these areas of the protein coagulum of the ventricular 

 cerebro-spinal fluid. 



In the higher-power figure (fig. 73) of the squared area from figure 72, the area 

 membranacea inferior shows the same character as exhibited by the specimen of 

 15 mm. (fig. 71). The opening maintains the same approximation to the lateral lip 



