IX.] THE PARASPHENOID. 235 



to form the maxillo-palatine (Fig. 71, pmx). The jugal 

 (Fig. 71, j) and the quadrato-jugals (Fig. 71, qj) are also 

 developed during this stage, and connect the hinder end of 

 the maxillaries with the quadrate ; they are both delicate 

 rod-like bones, the quadrato-jugal being the larger. 



The top of the cranium, which up to this* time has not 

 been covered by either cartilage or bone, begins now to be 

 closed in by parosteal osseous deposits taking place to form 

 the nasals, frontals, and parietals. Outside the prootic 

 region the squamosal also begins to be formed by a deposit 

 taking place in the mesoblast external to the perichondrium. 



In the base of the skull, there are three osseous deposits 

 to form membrane bones, which soon uniting constitute the 

 equivalent of the parasphenoid of osseous fishes. 



The first of these is the osseous deposit which takes place 

 in the mass of tissue, previously spoken of as the rostrum 

 (Fig. 71, rbs); this is connected behind with two osseous 

 deposits, the basisphenoidal ossicles, formed one on each side 

 in front of the pituitary space. The third deposit is the 

 large somewhat reniform mass of bone (Fig. 71, bt) called 

 the basitemporal, which forms one of the most conspicuous 

 features in the skull at this stage, and consists of two sym- 

 metrical halves. The basitemporals have not at this stage a 

 very firm connection with the base of the skull, but shortly 

 they become firmly united with it and with one another, and 

 thus the pituitary space becomes entirely closed over below 

 by bone. 



16. During the previous stage, most of the bones of the 

 face and skull had begun to be ossified, so that during the 

 next or fourth stage, that is about the end of the second and 

 the beginning of the third week of incubation, the chief 

 changes consist in the progressive ossification of the various 

 bones. Since it is not our purpose to enter fully into the 

 structure of the skull of the fowl but rather to confine our- 

 selves to its early development, this and the succeeding 

 stages will be described with greater brevity than the first 

 three. 



17. One most important change which occurs during this 

 stage, is the fenestration of the ethmo-presphenoid cartilage. 

 This cartilage (Fig. 70, ps) formed during the second stage 

 an unbroken plate, but towards the end of the third and 



