1889.] .NATURAL SCIENCES OF PHILADELPHIA. 23 



perusing pages of the most fascinating romance, and is led on step by 

 step, as " the thing " grows, and shapes, and matures, with an almost 

 irresistible passion for the marvelous story. Omitting those parts 

 which I have referred to above, and confining ourselves to the 

 structures which properly fall within the present section, we are 

 clearly shown how in the endocranium in its pristine membranous 

 condition there is laid down at its base in primitive cartilage the 

 pair of rods which are the ground plan of the future brain-case, the 

 harborage of that most powerful of all organs, the encephala. These 

 rods of the trabecules cranii, which behind embrace the notochord 

 (the parachordals), while anteriorly their segmentations become the 

 trabecules proper and enclose the primitive pituitary space. 



From these simple beginnings, part after part, grows and evolves, 

 until in due time we have before us the mature skull with all its 

 associated structures. At the appointed instant sense-capsules are 

 born and elaborated pari passu as the cranial moulding proceeds ; 

 and nerves and vessels burrow with precision through tracts and by- 

 ways long known to their kind in the ancestral types of the species, 

 guided by the ceaselessly acting laws of variation and evolution. 



Turning to the nether aspect of the basis cranii in our embryo Neo- 

 tomu fuscipes we find the foramen magnum (Plate I, Fig. 3,/. m.) to 

 be of a subelliptical' outline, a form retained probably throughout 

 life, as it obtains in the skulls of other adult Neotomas which I have 

 examined. The supraoccipital is still in two parts, the medial ver- 

 tical suture being very evident. It is, however, rapidly ossifying, 

 bone having advanced to the superior arc of the foramen, and no 

 doubt that early in the next stage the supraoccipital would be in 

 one piece (s. o.). 



The condyles show very prettily, and as structures developed by 

 the exoccipitals (e, o.) they are well started in the process of ossifica- 

 tion, though their ossific centres have not yet impinged upon either 

 the supra or basioccipital. Much cartilage is still to be found, both 

 above them and at their sides. 



Embedded in this material below, we are to observe just in front 

 of the occipital foramen the subquadrilateral form of the basiocci- 

 pital, already nicely started in bone. This osseous part does not as 

 yet reach the auditory bulla on either hand, though posteriorly it 

 arrives at the margin of the foramen magnum. Anteriorly, a car- 

 tilaginous tract intervenes between the concave border of the basi- 

 occipital and the ossific centre which represents the future basisphe- 



