THE SKULL OF A HUMAN FETUS OF 43 MILLIMETERS GREATEST 



LENGTH, 



INTRODUCTION. 



Since the publication of my work on the skull of a human fetus of 40 mm. 

 (Macklin, 1914) I have had the opportunity of studying and modeling a very satis- 

 factory specimen of a somewhat younger stage. This is human fetus No. 886 of 

 the collection of the Carnegie Institution of Washington. The sections were cut 

 in paraffin in the frontal plane at a thickness of 100 micra and stained with 

 alum cochineal. The technical work was all excellently done, and the series is 

 practically perfect. 



The models, of which there are 28, were made by the plaster-of-paris method 

 of Lewis (1915), which reproduces the structures with almost absolute accuracy. 

 The skull as a whole was first reconstructed at a magnification of 10 diameters and 

 the details were then worked out in separate models, most of which were made at a 

 magnification of 20 diameters. In the few instances where small details were lost, 

 these were made good by making new models. Each model was very carefully 

 checked by comparing each separate plate line, as the model was being painted, 

 with the bromide photographs of the sections. The models are, I believe, as nearly 

 exact reproductions of the original structures as it is possible to obtain. The bones 

 were modeled on the right side only. 



The parietal and frontal bones were too delicate to be modeled except in out- 

 line, and in drawing their texture the method of profile reconstruction was employed. 

 The same method was used to check up the general outline of the skull and to obtain 

 the relation of the external form and of the brain to the skull. 



The drawings were accurately made by Mr. J. F. Didusch. The method of 

 geometric projection was used, which insures an accurate representation of the 

 original. 



The study of No. 886 gave an opportunity for comparison of this specimen with 

 the skull of a 40 mm. fetus from Professor McMurrich's collection, known as 

 "I" Toronto," and hereinafter referred to as "la," which I previously modeled. 

 In la the measurement was crown-rump, and in No. 886 it was greatest length, and 

 this accounts for the fact that No. 886 is considerably younger though of greater 

 linear dimension. An opportunity was also afforded to compare the skull of No. 

 886 with that of No. 460 of the Carnegie collection, which has recently been 

 modeled by Lewis (1920) and which shows the condition of the skull in a human 

 fetus of 21 mm. 



Young cartilage and precartilage, although not abundant at this stage, were 

 included, and are mentioned where they occur. 



Terms of direction in the following description are all related to the basal plate 

 in a horizontal position. Accordingly they are, at times, at variance with the terms 

 of orientation applying to the adult skull, particularly in the ethmoidal region, since 

 this is usually described with the basal plate almost vertical. For the same reason 

 the terms are often different from those used in the description of la. 



59 



