SKULL OF A HUMAN FETUS OF 40 MM. 421 



The medial pterygoid plate (figs. 2, 3 and 10) is represented 

 by an elongated and somewhat spiral rod, lying behind the 

 palate bone and medial to the lateral portion of the ala temporalis. 

 Two distinct portions may be recognized in it, the upper part 

 being osseous and the lower cartilaginous. The former is bent 

 dorsally and from its most posterior extremity, which is about 

 3 mm. from the medial angle of the lateral portion of the ala 

 temporalis, a short but stout process projects cranio-medially, 

 marking the highest part of the rod. Below it terminates in the 

 cartilaginous hamular process, which points almost straight 

 downward. 



The membrane bone of the upper portion is typical in structure, 

 and when followed downward shows a gradual transition into 

 the cartilage of the hamular process. The cells of the membrane 

 bone enlarge, their capsules swell, and at the same time the 

 ground substance becomes lighter in color. Nearer the cartilage 

 which forms a cap to the extremity of the osseous rod the capsules 

 become smaller, the ground substance increased in amount, and 

 the line of transition is not sharply marked. Though the tip 

 of the cartilage partakes of many of the properties of normal 

 cartilage — staining lightly, having a homogeneous matrix, the 

 nuclei surrounded by capsules, and the whole being enveloped 

 by a closely-fitting, sharply defined, thin sheet of perichondrium 

 — yet there are several points of difference which mark out this 

 mass of cartilage as different from that found elsewhere in the 

 primitive skull. The matrix stains slightly more darkly, the 

 nuclei are larger and lighter in color, and the capsules surround- 

 ing them are relatively smaller, when compared with the size 

 of the nuclei. Altogether the cells resemble those of membrane 

 bone more than they do those of typical cartilage. 



The mandible (figs. 3-4) is a plate of membrane bone lying 

 immediately lateral to Meckel's cartilage, and separated from 

 this throughout its extent by connective tissue, except in a 

 small area in the region of the lateral incisor and canine tooth 

 germs, where, as has been noted, the bone is directly applied to 

 the cartilage, appearing like ossified perichondrium. The pos- 

 terior part of the bone is wider than the anterior, and is marked 



