ILUNOIS BIOLOGICAL MONOGRAPHS [8 



THE CHONDROCRANIUM OF THE 10 MM. LARVA 



The description of the eariy chondrocranium of Amiunis is based upon the 

 study of a group of specimens from 8 to 10 mm. in length. Earlier stages than 

 the 8 mm. larva were not prociu^ble. From one series of transverse sections 

 of a 10 mm. larva of A. nebulosus (catus), a wax model of the entire craniimi 

 and the visceral arches was made by Bom's method of reconstruction. The 

 description of this is supplemented by a careful microscopical examination of 

 several other specimens. 



The cranium is elongate and not as depressed as it is in the adiilt animal. ' 

 There is no roof of cartilage or of bone above the brain at this stage, although 

 a transverse bar between the alisphenoid cartilages of the two sides divides 

 the large space into an anterior and a posterior fontanelle (Fig. 2). The ante- 

 rior of these is the smaller and is limited to the region above the forebrain and 

 the olfactory region. The posterior fontanelle is very large and extends 

 posteriorly between the dorsal margins of the otic capsules, its posterior margin 

 is formed by the occipital arch. 



The floor of the craniiun is fenestrated by the large oval fenestra hypo- ' 

 physeos (f.h.), which extends from the anterior end of the parachordal plate 

 (pch) to the posterior end of the ethmoid plate (eth), the trabeculae cranii (/r) 

 forming its lateral margins. The parachordal plate is solid posteriorly and is 

 grooved for the reception of the saccvili of the inner ears. On either side aijd 

 above the parachordal plate the otic capsules are situated and the only foramina 

 in the wall of the cranium in this region are those for the passage to the exterior 

 of the glossopharyngeal (Fig. 1, DC) and vagus nerves (X), nothing comparable 

 to the posterior basicranial fenestra and the basicapsular fenestrae of Salmo 

 (Parker, 1872; Gaupp, 1906) being observed. The cavum of the lab\Tinth 

 opens widely into the cavmn cranii. The alisphenoid cartilage, extending ^ 

 dorsally and anteriorly from the anterior end of the otic capsule, forms the 

 dorsal margin of a large foramen in the cranial wall, through which the optic, 

 oculomotor, abducens, trigeminal and facialis nerves issue (Figs. 1, 2). The 

 nerves penetrate the connective tissue membrane which extends across the 

 fenestra from the alisphenoid cartilage to the trabecula cranii (Fig. 14). The 

 detailed descriptions of the various regions of the cranium and the comparisons 

 with the chondrocranial parts of other forms follows. 



The ethmoid region. At the 10 nun. stage in the development of the eth- 

 moid region, the dorsal surface of the cartilage forms a trough, the sides of 

 which are formed by dorsal projections near the lateral edges of the plate; 

 the floor is formed by the ethmoid plate itself (Fig, 2). The olfactory lobes 



