CHAPTER IX. 



THE DEVELOPMENT OF THE SKULL. 



1. In the chapter on the fifth clay, we gave a short 

 description of the earliest stages of the development of the 

 skull. The subject is however of sufficient importance to 

 merit a separate chapter, and in order to render the present 

 account complete in itself, we have found it necessary to 

 repeat a few of the statements already made. 



2. In its earliest condition the cranium is composed of a 

 mesoblastic tissue of stellate cells which can be distingriished 

 from the remainder of the mesoblast by its greater opacity. 

 In this condition (which is that of the fourth day), it may be 

 spoken of as the membranous cranium. From this mem- 

 branous condition the tissue composing it rapidly passes into 

 true hyaline cartilage. 



3. The primitive skull 1 , from its very first formation on 

 the fourth day, consists of elements which fall into two very 

 distinct divisions. We have on the one hand a sheet of 

 cartilasre which ensheaths the notochord from its anterior 

 end to the first vertebra. This sheet of cartilaq-e forms an 

 unsegmented continuation of the vertebral bodies. It is to 

 be considered as the most anterior portion of the axial 

 skeleton, in which the segmentation has become obliterated ; 

 aud as such is equivalent not to one, but to a (hitherto not 

 certainly determined) number of vertebrae. 



This sheet was spoken of by Rathke (Entivickelungs- 

 geschichte der Natter), its discoverer, as the 'investing mass' 



1 The facts narrated in this chapter are mainly derived from Mr Parker's 

 Memoir upon the Development of the Skull of the Common Fowl (Gallus 

 domesticus), Phil. Trans., 1866, Vol. clvi., pt. i. 



E. 15 



