RELATIVE WEIGHT AND VOLUME OF COMPONENT PARTS OF FETAL BRAIN. 47 



The first question that arose was the selection of a method of subdivision which 

 could be adapted to all stages and which would yet permit the greatest number of 

 dissections and comparisons. Only simple determinations were possible in the 

 early stages, additions being made as the encephalon increased in complexity. In 

 seeking to establish definite landmarks by which to be guided in delimiting the 

 various parts, it was decided to base the initial procedure upon the early subdivision 

 of the encephalon into the primary brain vesicles and then to add such later sub- 

 divisions as could be accurately determined by a study of the gross models and a 

 microscopic study of the tissues. This plan permitted the development of a logical 

 method of procedure. 



Since in all stages it was possible to delimit the three early vesicles the 

 rhombencephalon,mesencephalon,and prosencephalon the first point to determine 

 was that of landmarks by which these vesicles could be separated from each other 

 and the hindbrain from the spinal medulla. In the latter, of the three criteria 

 most commonly used the highest root fibers of the first spinal nerve, the lowest 

 root fibers of the hypoglossal nerve, and the lowest crossing fibers of the pyramidal 

 decussation the first proved to be the most satisfactory. Accordingly, the first 

 section was made in a transverse plane immediately anterior or cranial to the highest 

 rootlets of the suboccipital nerve. This section was the same in all cases, and 

 the resulting anterior mass, the encephalon, was the portion used for these studies. 

 The second section the separation of the hindbrain from the midbrain was made 

 in the youngest specimen (4.3 mm. CR length) in the constriction immediately 

 in front of the outward curve of the rhombic lip, the cerebellar rudiment. Later, 

 when it was possible to discern the inferior colliculi and the superficial transverse 

 pontine fibers, the incision was started dorsally, just posterior to the colliculi, to end 

 ventrally immediately anterior to these pontine fibers. 



The mesencephalon is much more prominent proportionally in the younger 

 subjects than in the older, and forms the arch of the cephalic flexure. It is wedge- 

 shaped, the ventral surface being concave and shorter, the dorsal surface convex 

 and longer, and delimited in front and behind by transverse surface grooves. The 

 posterior incision was comparatively easy and was made, as described above, to 

 remove the rhombencephalon. The line of demarcation between the mesenceph- 

 alon and the prosencephalon was, however, more difficult to follow, since it was 

 found upon studying the tissues that the thickened lateral plates of the mesen- 

 cephalon project forward into the prosencephalon or, more properly speaking, are 

 invaginated as a result of the caudal growth of the prosencephalon, so that the line 

 of incision must extend anteromesially between these mesencephalic plates and the 

 more prominent overlapping masses of the thalamencephalon. This lateral thick- 

 ening of the mesencephalic plates, which is more pronounced in the basal parts 

 ventral to the iter (the site of the future tegmental nucleus), is sufficiently well 

 marked in these early stages to prevent error. Later, other landmarks make their 

 appearance; dorsally, the epiphj r sis, the habenular nuclei, the posterior com- 

 missure, and the increasing prominence of the posterior extremities of the thalami; 

 anteriorly, the pulvinars; posteriorly, the differentiation of the superior colliculi; 



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