TRANSITORY CAVITIES IN THE CORPUS STRIATUM OF THE 

 HUMAN EMBRYO. 



By Charles R. Essick. 



INTRODUCTORY. 

 The human embryo, during the seventh week of development, possesses two bilaterally 

 symmetrical cavities in the substance of the corpus striatum. Since one lies nearer the 

 midline than the other, I have called them cavum mediale corporis striati and cavum 

 laterale corporis striati. The former is deeply placed in the striate body, while the latter 

 occurs nearer the surface of the brain, at times separated from the pia mater by a layer of 

 nervous tissue only a single cell in thickness. In all of the embryos in the collection of the 

 Carnegie Institution of Washington, ranging from 15 to 20 mm., crown-rump measurement, 

 one or both o: these cavities are present, while above these measurements they appear with 

 diminishing frequency; so that in all of the specimens under 15 mm. (measurement in for- 

 malin) and all over 24 mm. evidences of them are lacking. 



Undoubtedly the ephemeral character of these cavities and their similarity to artifacts 

 account for the fact that they have heretofore escaped notice. The study of human 

 embryological material can not always be followed in specimens that have been fixed alive ; 

 more often one must be content with tissue that has begun to macerate. It is very natural, 

 then, to regard a rent in tissue as a fault in technique, especially in the central nervous sys- 

 tem, where shrinkage is one of the most difficult things to avoid in preparing and mounting 

 serial sections. 



THE CELL-CONTENT OF THE STRIATE CAVITIES. 

 Inasmuch as the identification of both mesial and lateral cavities is based on their 

 contents, it may be well to describe the peculiar cells, foreign to the nervous tissue, which 

 occupy both cavities in large numbers whenever found in the striate body. They must be 

 regarded as the best proof of the normal occurrence of spaces in the human brain. From 

 the moment one of these striate cavities makes its appearance as an irregular break in the 

 continuity of the nervous tissue, until its complete disappearance, it is inhabited by large 

 amceboid cells having the power of phagocytosis. Study of the whole embryo and its 

 membranes shows that these cells are not peculiar to the cavities; for the same cellular 

 elements may be seen in the mesenchymal spaces, especially when of loose texture. Enor- 

 mous numbers may be found in the younger chorionic villi; along the umbilical cord many 

 wander near the surface; multitudes are present everywhere in the loose mesenchyme 

 around the central nervous system, especially near the base of the fore-brain; they occur 

 with less frequency along the aorta and in the mesentery. 



These widely distributed cells have been figured by many observers— usually in con- 

 nection with the formation of the blood. Hofbauer, Grosser, and Minot have illustrated 

 the cells in the chorionic villi of human embryos of the first months of development. Hof- 

 bauer (5, p. 28) first called attention to specific round cells appearing in the human placenta 

 toward the end of the fourth week of pregnancy. Describing in detail these "vakuolaren 

 Zellen," he called attention to the many points of similarity to the plasma cell— i. c, the 

 eccentric nucleus with its rich chromatin network and its surrounding clear zone. 



