lir 



memoir " On the Morphology of the Cephalous Mollusca," in which he 

 investigated the structure of these animals according to the same 

 canons which had guided him in respect to the Medusae. Other 

 papers, such as those on Doliolum, Sagitta, and Tethyia, were also the 

 outcome of his labours at sea, but part of the programme which he 

 sketched out in his letter to Sir W. Burnett remained for a long time 

 unfinished, and part was never completed at all. 



For new lines of inquiry were continually opening up to him. In 

 the first place he was occupying himself with morphological prob- 

 lems presented by invertebrate forms other than those which came 

 before him on the ocean, as shown by his paper on Aphis, pub- 

 lished in the ' Linnean Transactions ' in 1858, and by others ; and 

 with still greater energy did he throw himself into vertebrate morph- 

 ology, preparing himself for the task by a careful study of vertebrate 

 embryology, a subject then,, in spite of the lead given by Allen 

 Thomson, much neglected in England, though most successfully 

 cultivated in Germany by Kolliker and others. One of the fruits of 

 these labours was the Croonian Lecture, " On the Theory of the 

 Vertebrate Skull," delivered November, 1858, in which, following up 

 Rathke, he strove to substitute for the then dominant fantastic doc- 

 trines of the homologies of the cranial elements advocated by Owen, 

 sounder views based on embryological evidence. He exposed the 

 futility of attempting to regard the skull as a series of segments, in 

 each of which might be recognised all the several parts of a vertebra,, 

 and pointed out the errors of trusting to superficial resemblances of 

 shape and position. He showed, by the history of the develop- 

 ment of each, that, though both skull and vertebral column are seg- 

 mented, the one and the other, after an early stage, are fashioned on 

 lines so different as to exclude all possibility of regarding the 

 detailed features of each as mere modifications of a type repeated 

 along the axis of the body. " The spinal column and the skull start 

 from the same primitive condition, whence they immediately begin 

 to diverge." " It may be true to say that there is a primitive 

 identity of structure between the spinal or vertebral column and the 

 skull ; but it is no more true that the adult skull is a modified verte- 

 bral column than it would be to affirm that the vertebrate column is a 

 modified skull." This lecture marked an epoch in England in verte- 

 brate morphology, and the views enunciated in it, carried forward, if 

 somewhat modified, as they have been, not only by Huxley's subse- 

 quent researches and by those of his disciples, but especially by the- 

 splendid work of Gregenbaur, are still, in the main, the views of the 

 anatomists of to-day. 



In the second place, led probably by the desire, which only gradu- 

 ally and through lack of fulfilment left him, to become a physiologist 

 rather than a Daturalist, he turned to histological themes, as shown 



