EARLY STAGES OP DEVELOPMENT OP THE RABBIT. 115 



acid, &c., and sections have been examined of all stages from 

 the 30th hour onwards. 



A large portion of the work for the present paper, and the 

 three or four other papers I hope to publish shortly, was carried 

 on while I held the post of Demonstrator of Zoology in the 

 Owens College. 



To the kind consideration of the late Professor Arthur Milnes 

 Marshall I am indebted for many opportunities for work which 

 I should otherwise have found difficult to obtain. 



The remainder of the work has been done in the Morpho- 

 logical Laboratories of Cambridge through the permission of 

 Mr. Sedgwick, to whom I wish to express my gratitude for the 

 said permission and for his kindness in reading my papers and 

 offering many valuable criticisms. 



CHAPTER I. 

 Segmentation of the Ovum. 



Since the accuracy of van Beneden's account of the early 

 stages of the development of the rabbit depends to a great extent 

 upon the way in which the earliest cleavage planes succeed one 

 another, I think it advisable to give in some detail the results 

 of my own researches on this point. In fact the very first 

 segmentation division, according to van Beneden, gives rise to 

 an important question, which is as follows : is there any 

 essential difference between the two first segments, and do the 

 descendants of one segment give rise to the epiblast cells, and 

 the descendants of the other to the hypoblast cells ? 



To this van Beneden replied that there is always a difference 

 in size between the two first segments, and that the smaller 

 of the two is the more granular, and that from that one are 

 derived all the cells of the inner mass, and that the larger 

 clearer segment gives rise to cells which gradually surround 

 the descendants of the small segment and form the wall of the 

 future " blastodermic vesicle." 



From the descendants of the small segment van Beneden 



