xeii report — 1877. 



of being traced from one group to another ; this being in fact equivalent to the 

 statement that there is a similar type of structure pervading the animals 

 of each group, and a probability of a common type being ascertained 

 to belong to them all. The main question, therefore, to be answered is 

 whether there is or is not a general correspondence between the phenomena 

 of development and the gradation of type in animal structure upon which 

 anatomists and zoologists are agreed ; and my object will now be to bring 

 rapidly before you one or two of the most marked illustrations of the 

 correspondence, drawn from the early history of development in the higher 

 animals. 



As one of the examples of the earlier phenomena of development I may 

 refer to the change which is perceptible as early as the 18th or 20th hour 

 of incubation in the chick, and which is reproduced in the course of develop- 

 ment of every member of the Yertebrate subkingdom. It consists in the 

 formation of cross clefts on each side of the primitive neural cavity, which 

 divide off from each other a number of segments of this wall in the length 

 of the axis of the embryo. At first there are only one or two such clefts ; 

 but they rapidly increase in a backward direction in the body of the embryo, 

 and as development proceeds they extend into the tail itself. These 

 are the protovertebrce of cmbryologists — not corresponding, as might at 

 first be supposed, with the true or actual vertebra) which are formed later, 

 but representing in an interesting manner transverse vertebral segments 

 of the body, and containing within each the elements of the several 

 structures belonging to the body-wall afterwards to be developed, in- 

 cluding the true cartilaginous or osseous vertebral arches and the muscular 

 plates. 



This change, however, belongs to the mesodermic lamina, and occurs 

 in an elongated thick portion of it, which makes its appearance on each 

 side of the primitive neural canal between the epiblast and the hypoblast. 

 The transverse cleavage is ascertained to commence near what afterwards 

 forms the first cervical vertebra, but does not extend into the base of the 

 cranium. And it is most interesting to note in this cleavage the formation 

 at so early a period of the succession of metameres or series of similar parts, 

 which forms a main cb aracteristic of vertebral organization. 



As intimately connected with the formation of tho vertebral column, the 

 appearance of the chorda dorsalis or notochord presents many points of 

 peculiar interest in embryological inquiries. 



The notochord is a continuous median column or thread of cellular struc- 

 ture running nearly the whole length of the rudimentary body of the 

 embryo, and lying immediately below the cerebro-spinal canal. It occupies, 

 in fact, the centre of the future bodies of the vertebras. It exists as a pri- 

 mordial structure in the embryo of all Vertebrates, including man himself and 

 extending down to the Amphioxus, and, according to the remarkable discovery 



