Embryology 293 



EXPERIMENTAL EMBRYOLOGY 



The first studies in embryology were mainly concerned with the 

 description of developmental stages in various animals. At present 

 the most active phase of this work is concerned with learning more of 

 the details of the events of early human development. A second phase 

 of interest was that dealing with the comparison of the rates and modes 

 of development among various animals. Experimental embryology 

 is the third and most recent phase; at present, it is the aspect being 

 most actively studied. By means of carefully planned experiments, much 

 is being learned of the dynamics and the controlling mechanisms which 

 lie behind the embryological changes. 



Organization of the Egg. — During the discussion of early em- 

 bryology it was pointed out that there was organization in the tgg. The 

 vegetal and animal poles were well differentiated, and the cleavage planes 

 occurred in relation to this primary axis. Further, the rate of cleavage 

 was higher at the animal than at the vegetal pole and resulted in a 

 larger number of smaller cells. Apparently the right and left sides of 

 the future embryo are not so definitely established, and there is a possi- 

 bility that the site of entrance of the sperm may influence this symmetry. 



The question of whether the regions, which will later develop into 

 organs, are already set aside in the egg is one of considerable impor- 

 tance. By means of dyes which do not injure the tgg, it has been shown 

 that certain regions of the cell are destined to become specific organs 

 of the embryo. 



One protochordate, Styela, demonstrates this localization of organ 

 sites in a very vivid manner. Before fertilization the eggs of this animal 

 have yellow pigment granules scattered throughout the outer layer 

 of the cytoplasm. During fertilization these pigment granules concen- 

 trate, forming a yellow crescent at the area which will later be the 

 posterior surface. Opposite to this, at the area which will be the an- 

 terior surface, a gray crescent appears. In later cleavage, it is shown 

 that the yellow crescent is distributed to the mesoderm of the tail, the 

 gray crescent to the cells forming the notochord and neural plate, the 

 yolk to the entodermal cells, and the remainder of the egg to the cells 

 of the epidermis. 



If single blastomeres resulting from early cleavage of Styela are 

 isolated, each will develop only into that organ which would normally 

 form from that portion of the cytoplasm of the egg. This determinate 

 cleavage is characteristic of certain invertebrates. The vertebrates, on 



