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



Many workers have tested out various fixatives with more or less indifferent 

 success, and while for special tissues and under favorable conditions many satis- 

 factory methods have been developed, still for general purposes, where availability 

 of the agent and the ease with which it can be prepared are the main desiderata, 

 none have yet been discovered which are as satisfactory and consequently as 

 widely used as is formalin in varying degrees of strength. 



In order to secure the necessary data upon which to base conclusions, a great 

 number of embryos were studied and such stages selected as it was thought would 

 best show the developmental phases under consideration. Out of this number, 10 

 specimens were chosen, ranging in size from 4.3 mm. CR, estimated age 4^ weeks, 

 to a new-born infant of 367 mm. CR. All the material used is to be found in the 

 Carnegie Embryological Collection. In table 1 will be found listed the embryos 

 that were selected as being particularly suitable for the purposes of this study. 

 They are arranged serially in the apparent order of their development.- The 

 catalogue numbers given are those under which the embryos are listed in the 

 records of the laboratory. The measurements all signify the crown-rump length in 

 millimeters. The ages in weeks were determined from the last known menstrual 

 period and conform to the curve of age (based on length and weight) plotted by 

 Streeter (1920). All of the specimens had been preserved in formalin, and the first 

 8, in addition to fixation, had been embedded, cut, and stained. The material was 

 used as found, no attempt being made to account for any tissue changes dependent 

 upon the technic. The volume is therefore that of the embedded specimen. The 

 shrinkage resulting from the embedding process introduces a considerable error in 

 connection with actual volume, but as the shrinkage of the component parts of a 

 given brain is uniform, the error does not prevail in reference to the relative volume 

 of the separate parts. 



In each of these younger embryos the brain was projected, drawn, and modeled, 

 according to the well-known method of Born. (See table 1 for magnifications used 

 in each instance.) The resulting models could then be easily subdivided into the 

 various parts decided upon and the weights and volumes ot these readily obtained. 

 The two older specimens were of a sufficiently advanced stage of development to 

 permit of dissection and weighing of the actual tissue. 



TABLE 1. List of specimens the brains of which were studied. 



