TEXT-BOOK OF EMBRYOLOGY 



INTRODUCTION 



The study of human embryology deals with the development of the individual 

 from the origin of the germ cells to the adult condition. To the medical student 

 human embryology is of primary importance because it affords a comprehensive 

 understanding of gross anatomy. It is on this account that only recently a 

 prominent surgeon has recommended a thorough study of embryology as one of 

 the foundation stones of surgical training. Embryology not only throws Hght 

 on the normal anatomy of the adult, but it also explains the occurrence of many 

 anomalies; and the origin of certain pathological changes in the tissues. From 

 the theoretical side, embryology is the key with which we may unlock the secrets 

 of heredity, of the determination of sex, and, in part, of organic evolution. 



There is, unfortunately, a view current among graduates in medicine that the 

 field of embryology has been fully reaped and gleaned of its harvest. On the 

 contrary, much productive ground is as yet unworked, and all well-preserved 

 human embryos are of value to the investigator. An institute of embryology 

 for the purpose of collecting, preserving, and studying human embryos has re- 

 cently been estabhshed by Professor F. P. Mall of the Johns Hopkins Medical 

 School. Aborted embryos and those obtained by operation in case of either normal 

 or ectopic pregnancies should always be saved and preserved at once by immersing them 

 intact in 10 per cent, formalin or in Zenker's fluid. 



Historical. — The science of modern embryology is a comparatively new one, 

 originating with the use of the compound microscope and developing with the 

 improvement of microscopical technique. Aristotle (384-322 b. c), however, 

 centuries before had followed the general development of the chick day by day. 

 The belief that slime and decaying matter was capable of giving rise to Hving 

 animals, as asserted by Aristotle, was disproved by Redi (1668). 



A few years after Harvey and Malpighi had published their studies on the 

 chick embryo, Leeuwenhoek reported the discovery of the spermatozoon by 

 Ham in 1677. At this period it was beheved either that fully formed animals 

 existed in miniature in the egg, needing only the stimulus of the spermatozoon to 



