Orientation 



I. Some Definitions Relative to Embryology 



The word embryo has various shades of meaning. In general, it is applied 

 to the rudimentary or initial state of anything while it remains in an unde- 

 veloped or primitive condition. As used in zoology, it designates in one sense 

 the earlier stages of the development of an animal before the definitive or 

 adult form of the species is assumed; or, in a second sense, it signifies the 

 entire period of prenatal existence. 



The word development not only is used to denote the various changes evi- 

 dent in prenatal emergence, but also it applies to postnatal changes as well. 

 Moreover, in the development of a particular animal it may be extended 

 beyond the period of structural and physiological maturity to the changes 

 involved in eventual senescence. 



The developing young of viviparous animals while undergoing the later 

 stages of development within the uterus is spoken of as a fetus. This term is 

 used also, on occasion, to designate the later stages of development of oviparous 

 species. The phrase mammary fetus is applied to the young of marsupial mam- 

 mals such as the opossum while it remains attached to the nipple within the 

 marsupial pouch of the mother. 



The term descriptive embryology is applied to the method of embryological 

 study concerned with the direct observation and description of embryological 

 development. Up to the latter part of the last century embryology was con- 

 cerned mainly with the direct observation of the changes going on in the 

 intact embryo. However, beginning in the 1880's Wilhelm Roux and others 

 initiated the expeirmental approach in embryological study and the school of 

 experimental or causal embryology was formed. In experimental embryology 

 various parts of the developing embryo are removed, transplanted, parts are 

 exchanged, or the environmental conditions are altered. The end sought by 

 this method is an analysis of the respective roles played during development 

 by different parts of the developing organism and by different environmental 

 factors, in an endeavor to give a mechanical and functional explanation of 

 development. One of the outstanding results of the experimental method 

 applied to embryological study is the great body of evidence which points 

 to the fact that in the vertebrate group one of the main processes in devel- 

 opment (morphogenesis) is the induction of organs and organ-systems by 

 so-called organizer cellular areas present in certain parts of the developing 

 embryo. Organization of the developing body, in other words, is dependent 



