SKULL OF A HUMAN FETUS OF 40 MM. 327 



between the 40 and 80 mm. stages the posterior fossa has made 

 greater strides than the anterior, thus seeking to accommodate 

 the enlarging, backwardly-growing cerebral hemispheres. The 

 extent of development of the posterior portion of the brain- 

 case in the interval between the 40 and 80 mm. stages will be 

 realized when the ratio of the areas dorsal and ventral to the 

 hypophyseal fossa, as they are found in the model of Hertwig 

 and in my preparation, are compared. Although the dorsal 

 area exceeds the ventral in my model the excess is by no means 

 so great as it is in Hertwig's. 



Speaking generally, since the 28 mm. stage of Levi there has 

 been a flattening of the entire cranial floor. Between the 40 

 and 80 mm. stages the zone of greatest enlargement has been 

 the upper edge of the posterior cranial fossa, the effect being as 

 though this part had become stretched while the part around 

 the foramen magnum had remained relatively stationary. The 

 result is that the brain-case in this region has become more shal- 

 low and the sides more flaring, with their lateral and dorsal 

 surfaces directed more caudally than outwardly. The region 

 above the parietal plates has shared in this expansion, as is seen 

 by the more widely placed parietal bones in the model of Hert- 

 wig, and this period of development has also witnessed the flat- 

 tening out of the angle from the cranial aspect of the otic capsule, 

 as may be seen when these skulls are compared. 



Other expansive changes which have taken place in the interim 

 between the 40 and 80 mm. stages are noted in the region of the 

 middle ear, in the floor of the mouth, and in the temporal and 

 zygomatic fossae, the details of which will be taken up in the dis- 

 cussion of the regions. In general one is struck with the large 

 development of the cranial cavity, which gives to the Hertwig 

 model a relative broadness when seen from the front, and has 

 also resulted in a greater preponderance of the size of the cranium 

 when compared with the facial region. A comparison of the 

 Hertwig model with that of Jacoby for the 30 mm. stage brings 

 out an even greater disproportion. 



Of the skulls of other mammals those of Macacus cynomolgus 

 and Semnopithecus maurus, modelled by Fischer ('03), may be 



