SKULL OF A HUMAN FETUS OF 40 MM. 331 



purple, thus standing in sharp contrast to the bluish matrix of 

 the cartilage. This mass appears to be opposite the first part 

 of the sub-basal course of the chorda dorsalis, and lies just ventro- 

 caudal to the middle of the long axis of the planum. It belongs 

 to the occipital portion, and is separated from the perichondrium 

 by a thin sheet of mature cartilage. The perichondrium in the 

 vicinity of this mass is unchanged, and contains no osseous 

 tissue. In the 28 nmi. stage of Levi the cells were, in this area, 

 very large, and were surrounded bj^ a very prominent capsule. 

 We have in this area, no doubt, the beginning of the endochondral 

 ossification center of the basi-occipitalis, which is said by Mall 

 ('06) to appear on the 65th day. 



The course of the chorda dorsalis has become familiar through 

 numerous investigations, and I have nothing to add concerning 

 it. The condition which I find agrees essentially with that 

 illustrated by Fawcett for the 21 mm. stage (Fawcett '10a, fig. 2), 

 The site of the future ligamentum suspensorium dentis, at the 

 apex of the intercondyloid incisure, is marked by a coiled con- 

 dition of the notochord, which corresponds to an intervertebral 

 disk. 



In my model the caudal portion of the planum is but slightly 

 wider than the cranial, a condition which stands in sharp contrast 

 to that found in the model of Hertwig, where one notices that, 

 though the entire planum has undergone a widening, the region 

 bordering the primitive foramen magnum has outstripped the 

 more cranial portion in this regard. This broadening has, of 

 course, separated the caudal extremities of the ventral and 

 dorsal basicochlear grooves. The latter are both shallower 

 and less strongly marked than in my model, as are also the 

 sphenocochlear notches. Further, the side-to-side concavity 

 in the model of Hertwig is almost obliterated, as is also that 

 from above downward, and the inclination of the planum in the 

 latter model is less steep than in mine. 



