1879.] Skull and its Nerves in the Green Turtle. 331 



tion to the notochord was considered by me to be that of direct obstruc- 

 tion to the forward growth of that rod. 



I now, see, however, that this azygous structure, the true skeletal 

 axis, which is early separated from the hypoblast, begins to starve at 

 its fore end, before the pituitary body is formed. 



The main part of the hypoblast, as a rule, is arrested just behind 

 the oral involution ; and above that point the notochord ceases to 

 grow. 



At first, as Mr. Balfour shows, this rod is hooked downwards in 

 front, for it follows exactly the curve of the mid-brain. 



There is, however, at the time of its arrest, no physical obstacle to 

 stop its growth still further forwards beneath the fore- brain, to the 

 utmost limit of the frontal wall of the embryo. 



Thus the primary skeletal axis stops and shrinks by what may be 

 called an anticipation of the obstructing wall that will be. 



Here is something in morphology resembling hereditary instinct in 

 zoology. 



The segmentation of the embryo is at first altogether that which is 

 seen in the somatomes, and after these have become converted into 

 the muscle-plates a new alternating segmentation takes place, so that 

 the short muscular bands can work and produce the vermicular con- 

 tractions of the embryo. 



The tissue which becomes the vertebras is primarily marked out into 

 serial parts in conformity with the segmental cell masses that form 

 the muscle-plates. 



Very soon, however, a new segmentation takes place, and the 

 primordial vertebras are intermediate to the muscular masses. 



This secondary, intercalary vertebrate segmentation is very slightly 

 developed in the head. 



For a good while in all the Vertebrates, and permanently in some, 

 the mesoblastic sheath of the notochord becomes a continuous carti- 

 lage ; in all but the lower forms this undergoes segmentation to form 

 the "bodies " of the vertebras. 



In the head, as a rule, this second sheath of the notochord is but 

 little developed, and has a very slight degree of separateness from the 

 investing paired cartilages — para-chordals, and hinder part of the tra- 

 becule. 



In the Selachians, however, it is well developed, and in them the 

 distinction between head and body by means of the occipito-atlantal 

 articulation is late ; in some Batrachians, notably in the huge tadpoles 

 of Pseudis paradoxa, before the limbs are grown, this cartilaginous 

 sheath of the notochord is large and thick. 



The distinction between the hind and fore parts of the skull is 

 greater by far than the distinction between the vertebral column and 

 the hind part of the head. 



