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Mr. W. K. Parker on the Development of the [Feb. 13, 



For this latter region has, in common with the spine, the notochord, 

 its mesoblastic sheath, paired neural and haemal elements, besides a 

 hypoplastic lining to the digestive tube beneath. 



Moreover, there is a great tendency to produce vertebrae in the hind 

 part of the head in some Vertebrata ; in several of the Urodelous Am- 

 phibia there are three rudimentary vertebrae in front of the "atlas." 



Thus the scant growth of the hypoblast in the cephalic region of 

 the blastoderm, would appear to be one of the causes of the extreme 

 modification of the head as compared with the spinal column.* 



We shall have made a great stroke in embryology when we have 

 explained the peculiar behaviour of the epiblastic covering of the fore- 

 part of the head. 



There, the oral involution, which is formed from epiblast, turns 

 inwards and upwards into the fore-part of the hollow under the mid- 

 brain, and grafts itself upon the back of the down-turned fore-brain, 

 itself of epiblastic origin ; thus the pituitary body is formed. 



In all available interspaces the mesoblast of the fore half of the 

 head grows in and forms the supporting structures, and the vascular 

 supply. 



But now there arises this question — are the mesoblastic structures 

 of the cranium in the £>ro-chordal region perfectly homologous with 

 those in the para-chordal, whether cephalic or spinal ? 



In endeavouring to answer this question, I m ast return to the point 

 at which I started, where it was mentioned that there existed an un- 

 paired pro-chordal cartilage between the symmetrical trabeculae. 



This, in the fore-part of the chondro-craniiim, is a familiar part — 

 pre-nasal rostrum (" Ostrich's Skull," Plate 7, p, n) ; its largest 

 development is in the skate, saw-fish, and whale. 



But the perpendicular ethmoid and septum nasi are, in reality, other 

 parts of the same azygous cartilage (see " Frog's Skull," Plate 6, 

 figs. 9, 10) ; in the Batrachia I have studied the development of this 

 median part in a large number, both of individuals and species. 



But its real character is best seen in my second and third stages of 

 the embryos of Ghelone midas ; these measured along their curves are 

 respectively two-thirds of an inch and one inch and a quarter in 

 length. 



In the younger of these the trabeculae are like those of the frog 

 and Selachian, but they stop short and end in a somewhat out-turned 

 "cornu" behind the nasal sacs; they are flat in front and rounded 

 behind. 



* Since the above was written, Dr. Milnes Marshall has shown me that in the 

 embryo of the salmon the hypoblast, at first, runs forwards to the nasal sacs, and 

 ends in a blind cavity behind the frontal wall in the fore part of the palate. One of 

 my own figures shows this (" Salmon's Skull," Plate 2, fig. 10) ; I noticed and 

 figured this upper pre-oral recess, but could not interpret its meaning. 



