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



this by the weight of the part under consideration. The cubic centimeter value of 

 the wax being thus obtained in grams, it was a simple matter to calculate the 

 various amounts, percentages, etc. The actual volume was obtained by dividing 

 the model volume by the cube of the magnification used in the reconstruction. 



In making this study the companion parts of the two sides have in every 

 instance been grouped together, thus giving one unit for comparison instead of 

 studying each of these paired structures separately. In all cases, too, the percent- 

 age weight has been chosen as the unit of comparison, since this method gives a 

 stable basis, whatever may have been the magnification used in the reconstruction, 

 and would apply equally well when the actual weights were considered, as in the two 

 dissected specimens. Weight, rather than volume, has been selected for comparison 

 for the same reason that model weights were considered instead of attempting to 

 calculate actual weights, since it was thought that by reducing the method employed 

 to the simplest possible working basis the percentage of error would likewise be 

 reduced to a minimum. It is to be remembered, also, that these percentage 

 weights are of only relative value, for the various parts of the encephalon present a 

 steady and consistent growth and development throughout the period of gestation, 

 and that the decrease in percentage weight, recorded for certain parts, as opposed to 

 increased percentages for others, means merely that those parts which present an 

 increase in weight grow so much more rapidly that they outstrip their more 

 sluggish neighbors in relative bulk, as will be seen in a study of both models and 

 tissues. 



In every case, except the two oldest specimens, a careful check was kept upon 

 all steps by a microscopic study of the tissues. This was found to be especially 

 valuable in determining the landmarks to guide the incisions. However, while 

 occasional reference may be made to the degree of differentiation, it is the intention 

 at this time to deal entirely with the question of growth in bulk or mass, without 

 reference to histologic detail. 



DESCRIPTIONS OF INDIVIDUAL SPECIMENS. 



No. 148, 4.3 mm., estimated menstrual age 4^ weeks. 

 This embryo (see table 3 and fig. 1), which has 

 been the subject of a number of studies by various 

 investigators, presents some quite unusual 

 features and was selected for the reason that, for 

 a young stage, it presents the various develop- 

 mental features to such a degree as to render it 

 especially valuable as a basis upon which to build 

 a serial study of the various parts of the en- 

 cephalon. At this stage the central nervous 

 system is distinctly tubular in character and the 

 anterior neuropore is closed. Anterior and dorsal 

 to the eye-stalks, and separated from them by a 

 distinct fold, can be seen the olfactory region or 

 archipallium, and more dorsally the neopallium, 

 consisting at this stage of two faintly outlined 

 evaginations, one to either side of the mid-line. 

 The entire prosencephalon comprises 31.3 per 

 cent of the weight of the encephalon and can be 



quite easily subdivided into the telencephalon, 

 weighing 7 per cent, and the diencephalon, 

 weighing 24.3 per cent. The further subdivision 

 of the telencephalon yielded an archipallium of 

 4.2 per cent and a neopallium of 2.8 per cent. 

 The cephalic flexure is well defined; its arch, 

 rather narrow, is formed by the mesencephalon, 

 which constitutes 14.3 per cent of the encephalon. 

 The rhombencephalon at this stage is propor- 

 tionately very large, comprising 54.4 per cent of 

 the total weight of the encephalon. 



A study of this vesicle shows that the lateral 

 plates have a wide dorsal spread; the thin roof 

 plate is closely applied to the overlying ectoderm 

 and only a slight thickening is observed at the 

 point of junction of the roof and lateral plate 

 the rhombic lip. The cerebellar rudiment is 

 merely a thickened fold which stretches across the 

 more anterior, cephalic portion of the dorsum or 



