DEPARTMENT OF EMBRYOLOGY. 113 



gut. He found very few in the liver, the mesenephros, or the meta- 

 nephros, although they are abundant in the walls of the Wolffian duct. 

 There are some present in striped muscle and in the amnion. The 

 method was unsatisfactory for the examination of the spinal cord and 

 the brain, although clasmatocytes seemed to be present in the region 

 of the choroid plexus and were numerous in the pia arachnoid. In the 

 eye a few were observed in the sclera, but none in the retina or in the 

 choroid coat. 



Using cultures and fixed preparations of the subcutaneous tissue, 

 Mr. H. W. Vance has studied the finer structure of these same cells. 

 He has published a preliminary account of his observations, in which 

 he devotes particular attention to the centrosphere. 



INDIVIDUAL SYSTEMS. 



There have been completed four studies dealing with the central 

 nervous system and special sense-organs, and one each concerning the 

 lympho-vascular system and skeletal system. 



In a study previously published by me on the development of the 

 cartilaginous capsule of the ear in human embryos, I pointed out that 

 the changes in size and form which the capsule undergoes during its 

 development are accomplished, not only by progressive differentiation, 

 but also in part by a retrogressive differentiation of its constituent 

 tissues. The fact that certain areas of cartilaginous tissue revert to an 

 earlier embryonic type and are subsequently redifferentiated into a 

 tissue of widely different histological character, as in the case of the 

 otic capsule, is a factor of much significance. Such a process of retro- 

 gressive change, combined with redifferentiation of the same tissue 

 greatly increases the facilities for and the range of certain structural 

 adjustments that occur in many regions in the development of the 

 human embryo. Reference was made in the last report to the work 

 done in this laboratory by Professor Kunitomo, in which he shows 

 another instance of the same process in the case of the tail and caudal 

 end of the spinal cord in human embryos. Working on the same 

 structures, it occurred to me that it would be possible to determine in 

 the formation of the filum terminale to what extent we are dealing with 

 the dedifferentiation of the caudal end of the medullary tube, and to 

 what extent mth mechanical disproportion between the growth of the 

 medullary tube and that of the vertebral column. 



In younger stages the spinal cord and the vertebral column lie along- 

 side of each other in a metameric manner, corresponding in position 

 segment for segment. Owing to their disproportion in growth, there 

 occurs a relative displacement of their segment-levels; for instance, the 

 thirtieth segment of the cord comes to lie opposite the twentieth seg- 

 ment of the vertebral column. The segment levels of the vertebral 

 column are, of course, evident; in the spinal cord they are just as 



