30 STUDIES ON PATHOLOGIC OVA. 



angiology, 16 on splanchnology, 15 on the genito-urinary system, and 7 on the 

 coelom. The study of brain morphology is, to a certain extent, a science in itself, 

 and there are 24 papers on neurology. Only 9 publications on histogenesis have 

 appeared, but the histogenetic standpoint is considered in many of the other 

 papers enumerated above. 



As the collection grew we found an increasing number of specimens which, 

 though peculiar in form, were at first believed to be normal, but which upon closer 

 study proved to be pathological. Thus we were unwittingly carried into the field of 

 abnormal development. In fact, the great problem which confronts us always, in 

 the study of a new specimen, is to determine whether or not it is normal. The 

 experience of embryologists elsewhere has apparently been identical with our own, 

 for frequently one observes in the literature an account of a human embryo, 

 believed at first to be normal, which, upon further consideration, proved to be 

 pathological. Hochstetter has stated that we have no right to consider an embryo 

 normal unless it has been removed by a surgeon at hysterectomy, but our studies 

 along this line have shown that a quite appreciable number of hysterectomies 

 disclose pathological embryos. The criterion of His, who used the comparative 

 method of von Baer in determining the normality of human embryos, is probably 

 more reliable than that of Hochstetter. I believe, therefore, that the best check 

 in the study of the human is a knowledge of comparative embryology. 



There are 14 papers in the list that deal with pathological embryology. The 

 first (No. 21) included all specimens up to No. 162, which were believed to be 

 pathological. As we have found the two fields closely related, in these studies 

 we have naturally drifted from pathology into teratology, there being 10 papers 

 on this subject. 



Of the 500 papers emanating from the Department of Anatomy of the Johns 

 Hopkins Medical School, there are (in addition to the above-mentioned 159 dealing 

 with human embryology) 115 on experimental embryology. A study of human 

 embryology can not be of great scientific significance unless it be extended through 

 comparative studies and an effort be made to determine, if possible, the causal 

 relations between the developing parts. From the beginning, therefore, experi- 

 mental studies have been made upon a great variety of subjects, the most important 

 being on organ-forming substances and the influence of the eye-vesicle upon the 

 development of the lens from the ectoderm. We may also at this point call atten- 

 tion to the brilliant studies of Harrison upon the development of the nerves, 

 which demonstrate conclusively the. validity of the neuron doctrine. In making 

 these experiments he developed an ingenious method by which the living isolated 

 cells could be seen under the microscope and their growth followed during a number 

 of days. Later, this work was extended to the study of growth of all kinds of 

 isolated tissues in the warm chamber, now well known as tissue culture. 



In nearly all studies on embryo-anatomy it is necessary to resort to methods 

 which will enable one to see the structure and form of organs in their serial sections. 

 As far as possible this is always done by observing the whole embryo or parts of 

 one dissected under the dissecting microscope. A valuable specimen, however, 



