STRUCTURE OF THE VERTEBRATES 315 



powerful influence than the posterior portion. The removal of a 

 fragment from the posterior part of an early embryo may affect 

 the immediate surroundings; but the removal of a similar bit 

 of cells from the anterior region will carry its influence through- 

 out wdde areas. As organs begin their development the organiz- 

 ing influence becomes more specific, both in regard to the extent 

 of the area affected, and the reaction initiated in the neighbor- 

 ing structures. One or two cases will illustrate the principle 

 involved. 



During the development of the eye, the lens and the optic cup 

 develop sjmchronously; but if the epidermal covering over 

 the optic vesicle is removed, and epidermis from any other 

 region of the animal's body is grafted in its place, a lens will 

 develop in the normal position. The organizing substance which 

 causes lens development is apparently not limited to the cup 

 itself, but is present in the optic region of the brain. The con- 

 verse of this experiment is the transfer of tissues carrying the 

 more dominant organizator. The pelvic anlagen of early am- 

 phibian larvae can be removed and grafted against the vertebral 

 column in a more anterior position. These anterior vertebrae 

 will then develop sacral structures. 



From the above it will be seen that there are two influences 

 in the development of the organism as a unified whole: (1) the 

 inherent potentiality of cells and tissues to develop along spe- 

 cific lines; and (2) the modifying influences of one tissue upon 

 another. The nerves always arise from ectoderm, and the central 

 nervous system from a limited region of the ectodermal cover- 

 ing; but the position and structure of the nerves are influenced 

 by the mesodermal somites. Thus, in the growth of organs there 

 is a definite interdependence between tissues, causing an orderly 

 growth of the organism and its structures. 



The third regulator of development, the ductless glands, do 

 not function until the embryo is well formed. On the other hand, 

 there is an imperfect line of demarcation between the organizing 

 substances given off by tissues and the endocrine organs. The 

 latter are usually limited to those glands which secrete specific 

 hormones, and throw their secretions directly into the blood 

 stream. These internally secreting glands develop synchronously 

 with the other organs, but it is unknown when, during the course 



