CHONDROCRANIUM OF A 20 MM. HUM.A.N EMBRYO 607 



formation of the sloping areas cranially and dorsally, which is 

 initiated at the 17 mm. stage and reaches its completion at the 

 20 mm. stage. The second process is a rotation of the whole 

 basal plate on a transverse axis, resulting in the elevation of its 

 cranial end. This occurs between the 20 and 28 mm. stages. 

 The embryo reconstructed by Macklin (40 mm.) showed little 

 change in this respect from the 28 mm. stage of Levi. Accord- 

 ing to Levi, the basal plate is formed from two anlagen, occipital 

 and sphenoidal. These are triangular plates of cartilage ar- 

 ranged in siich a way as to meet with apices at what will be the 

 center of the basal plate. The base of the occipital anlage, that 

 is the side of the triangle opposite the angle meeting the sphenoid, 

 faces caudad and borders on the foramen magnum. The base of 

 the sphenoidal anlage faces craniad and fuses with the sphenoid 

 body at the crista transversa. The apices of these two trian- 

 gular cartilages are joined at first only by connective tissue. In 

 the 14 mm. stage, the cartilaginous fusion has just begun. In 

 the 17 mm. stage, it is well established though slender. In the 

 20 mm. stage it is complete, and the original separation can be 

 made out in the model only by the thinness of the cartilage. In 

 the sections it is easily traced by the manner in which the occipi- 

 tal and sphenoidal regions of the basal plate form their junction, 

 this beginning as a slender process ectally and becoming thicker 

 as the sections are followed toward the cranial cavity, (figs. 4 

 and 5). 



The completion of the basal plate and its fusion to the 

 sphenoid body takes place before there is any cartilaginous otic 

 capsule. The junction of the otic capsule to the basal plate be- 

 gins in the 17 mm. stage (Levi), when the latter is already com- 

 plete. This fusion is initiated cranially in the sphenoidal re- 

 gion, and progresses caudally. In the 40 mm. stage, Mackhn 

 could still trace a fine of separation caudally by the arrangement 

 of the cartilage cells. In the 20 mm. stage, as shown by the 

 sections, the fine of union can be traced even cranially (fig. 5, 2) . 

 No separation of basal plate from sphenoidal anlage was found 

 by Levi as early as the 13 mm. stage. This early fusion of the 

 sphenoidal portion of the basal plate to sphenoid body led him 



