106 TRANSITORY CAVITIES IN THE CORPUS STRIATUM. 



THE DISAPPEARANCE OF THE CAVITIES. 



The inoccupation of both cavities by nervous tissue and their consequent disappearance 

 is apparently as rapid as are their appearance and growth. Embryo 460 (21 mm.) shows 

 an extremely small cavity in the medial position on the right side and a complete absence 

 on the left. In no instance could evidence of the mesial cavity be found after it had ceased 

 to exist as a break in the nervous tissue, i. e., in embryos greater than 24 mm. crown- 

 rump length. An extremely interesting picture is furnished by embryo 453 (23 mm.), 

 inasmuch as it constitutes a stage immediately following the replacement of nervous tissue 

 in the cavum laterale. Close to the surface of this brain, where in younger stages one is 

 wont to find the lateral cavities of the striate body, a considerable collection of macrophages 

 lies among the nerve-cells. There is no longer any break in the nervous tissue, but, in a 

 very circumscribed area, these phagocytes may be made out where the walls fused with 

 one another. Large vacuolated cells are present in symmetrical positions on both sides 

 of the brain and are undoubtedly beginning to migrate from the nervous tissue. The 

 mesenchyme without the brain contains many of these same cells, which are present in 

 greater numbers near the site of the former cavities. It is not improbable that the macro- 

 phages which we see here distributed among the nerve-cells would have crept out to join 

 their brothers outside of the external limiting membrane. 



Three specimens included in this list are worthy of notice, inasmuch as they constitute 

 the extreme variations which have occurred. In the first, embryo 240, although its meas- 

 urement is recorded as 20 mm., no sign of either space can be made out. The tissue is 

 fairly well preserved and mounted. We are most likely dealing with a precocious or 

 delayed development of the cavity. On the other hand, we may have a condition analogous 

 to that obtaining for the pig. In this animal but two laboratory specimens, measuring 

 19 mm., showed an undoubted cavity in the corpus striatum (the mesial one) containing 

 macrophages. Many serial sections of pig embryos from 10 mm. to 25 mm. were searched 

 for evidences of either cavity, but in order to separate possible artifacts — which often occur 

 in the central nervous system — from a true cavity, a typical vacuolated cell with eccentric 

 nucleus was sought within the cleft. In all probability the existence of this cavity is of 

 extremely short duration in this animal. 



Embryos 128 (20 mm.) and 455 (24 mm.) showed a most remarkable degree of vacuoli- 

 zation of the striate body. The former embryo has a complicated cavity in the corpus 

 striatum, which lies just under the ependymal zone between the mesial and lateral cavities. 

 I have called this the cavum intermedium corporis striati, but it is evidently an anomaly. 

 It is unconnected with either the medial or lateral cavity, which are well developed. The 

 latter specimen shows a remarkable reduplication of the medial cavity. The region occu- 

 pied by the medial group is very much larger, being extended laterally in the ependymal 

 zone. There are so many small and independent spaces in the nervous tissue that it would 

 be tedious to count them. In most of these cavities are found one or more macrophages, 

 and their presence eliminates artifacts due to rapid dehydration. 



