SKULL OF A HUMAN FETUS OF 40 MM. 351 



In each successive stage the tips of the neural arches, both occipi- 

 tal and cervical (which retain their original alignment), are seen 

 to be farther advanced than in the last in their enclosure of the 

 spinal cord, a condition strikingly brought out by a comparison 

 of the foramen magnum and the underlying vertebral arches of 

 my model with those of the oldest Levi model, on the one hand, 

 and the Hertwig model on the other. In the 28 mm. Levi model 

 the tips of the neural arches of the occipital and upper cervical 

 vertebrae are separated by a considerable interval, in my model 

 they are almost united, and in the Hertwig model, as has been 

 noted, they are all completely joined; in the adult bone the tips 

 of the occipital vertebra are represented by the internal and 

 external occipital crests (representative of a spinous process). 

 This closure of the foramen magnum takes place, accordingly, 

 somewhere between the 40 and 80 mm. stages in man, and it bears 

 a striking resemblance to that of the segments of the spinal canal. 

 Growth seems to progress uniformly throughout the series, and 

 dorsal closure is apparently completed at about the same time 

 in each segment. Thus, with the fusion of the dorsal foraminal 

 prominences there, is completed what amounts to the closure 

 of the cranial extremity of the spinal canal. 



From what has been said regarding the formation of the fora- 

 men magnum it will be evident that what is found in the 40 mm. 

 stage is something more than the foramen of the adult condition; 

 it is this plus the superior occipital incism*e. Further, the 

 structure described as the tectum posterius is not the dorsal 

 delimitation of the real foramen magnum at all, but merely that 

 of the superior occipital incisure. The edges of the latter unite 

 in the form of a median seam upon the union of the dorsal extrem- 

 ities of the neural arches of the occipital vertebra, and thus is 

 effected the closure of the portion of the floor of the occipital 

 region dorsal to the foramen magnum. This conception of the 

 development of the foramen magnum explains why the primitive 

 foramen is relatively so much larger than the adult condition. 



In the condition of the occipital anlage at birth we find a 

 basilar portion, formed in its cranial part from the body mass 



