DEVELOPMENT OF THE SKULL IN THE PIO. 



291 



looks like hyaline cartilage, but the cells are crowded, and it is formed into bone before 

 the intercellular substance has time to appear*. 



Id ^e Mammal, more than in any other type, the original parts are all the more 

 orr Jtely transformed, in that the bony substance formed in the primordial cartilage 

 becomes very large in relation to its first model ; and, moreover, the " investing bones " 

 formed in the subcutaneous web become very large indeed, as compared with the small 

 granular territory, the soft model in which they first appeared. 



Also in no other type do the primary facial rods become segmented, arrested, and 

 metamorphosed to the same degree as in this the highest vertebrate Class. 



First Stage. — Embryo Pig, 7| to 8 lines long. 



The primordial, skeleton of the most highly specialized Mammal is as simple as that 

 of the lowest brain-bearing Fish ; the form of the fcetal head (Plate XXVIII. fig. 1) may 

 be aptly compared with that of the Fish and Frog (see my former papers on those types). 

 In embryos 1\ lines in length the three brain-vesicles (C 1", C 2, C 3) are hollow, 

 the film of soft brain-substance merely lining the enclosing membranous cranium. The 

 foremost of the vesicles has budded into the two rudimentary hemispheres above the 

 primary sac, the " thalamencephalon " (Plate XXVIII. figs. 3 & 6, C 1, C 1"); yet at this 

 stage the cutis does not cover the whole of the third vesicle (C 3) nor the whole of the 

 auditory sac {au.). The head is bent over upon the thin-walled thorax, and the cervical 

 region of the spinal chord is very outbent and swollen (fig. 1). 



The Visceral Clefts. — After noticing the brain-vesicles and the three pairs of sense- 

 capsules (ol.,e., au.), the foundations of which are already well laid, the eye detects that 

 peculiar dehiscence of the facial wall, the continuous face being cloven by the formation 

 of a series of slits or cuts, which pass quite through the substance of the cheek and neck. 

 By the older embryologists these are counted from behind the mouth ; but in my last 

 paper, especially, I have shown that the mouth itself is a great, double, completed cleft, 

 and that there is a secondary cleft in front of it, the " palato-trabecular " or preoral cleft 

 (cl. 1). But the " first postoral " is in reality the second cleft ; this is the largest in the 

 embryo pig, with the exception of the mouth. Behind this there are three others; and 

 the first of these, the " second postoral," is the counterpart of the most anterior of those 

 through which the water-currents pass in the osseous fish. Below and behind the 

 clefts the fore limb is seen in rudiment. Here it will be seen that there is a deficiency 

 in the number of clefts behind, as compared with the gill-bearing vertebrates (see papers 

 on Frog and Salmon). Only the first " postoral " cleft is persistent and functional, the 

 three behind soon closing in. I shall describe, anon, what becomes of the persistent 

 clefts, that in front of and that behind the great mouth-cleft. Between the clefts are 

 formed the arches ; these facial bars have some resemblance to ribs, but are formed 

 independently of axial parts, whereas the ribs are evident downgrowths from the ver- 

 tebral portions of the " Somatomes." 



* See on this subject, " On the Connective Tissues," by A. Rollett, in Stricker's 'Human and Comparative 

 Histology,' translated by H. Power for the New Sydenham Society. London, 1870, pp. 47-146. 



2 q2 



