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 light on the normal anatomy of 

 the adult, but it also explains the occurrence of many anomalies and mon- 

 sters, and the origin of certain pathological changes in the tissues. Obstet- 

 rics is essentially applied embryology. From the theoretical side, em- 

 bryology is the key with which we may unlock the secrets of heredity, 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 un worked, 

 and all well-preserved human embryos are of value to the investigator. 

 Only through the co-operation of clinicians in collecting and preserving 

 embryos will our ignorance of early human development be rectified. At 

 present, practically nothing is known of the maturing ovum, while of 

 fertilization, cleavage, and the formation of the germ layers we are en- 

 tirely in the dark. Aborted embryos and those obtained by operation in 

 case of either normal or ectopic pregnancies should always be saved and pre- 

 served at once by immersing them intact in 10 per cent formalin or in Zenker's 

 fluid. 



Historical. The science of modern embryology is comparatively new, 

 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 living 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 sperma- 

 tozoon by Ham in 1677. At this period it was believed either that fully 



