CELLULAR DIFFERENTIATION AND INTERNAL ENVIRONMENT 



79 



arthropods proceed through their early de- 

 velopment as syncytia without cell mem- 

 branes, the cells in other embryos are from 

 the first sharply divided from one another. 

 There may be regional cytoplasmic differ- 

 entiations in single cells that are as sharply 

 defined as cells are from one another, but 

 as division proceeds the various regions 

 become separated from one another by the 

 cell walls. 



Certain tissue elements, such as striated 

 muscle fibers in vertebrates and arthropods, 

 are multinucleate and surrounded by a 

 common membrane. Connective tissue cells 

 are often described as forming a syn- 

 cytium, but their ability to move about in- 

 dependently and the difficulty of resolving 

 optically the delicate connections between 

 fine cell processes invite caution in accept- 

 ing this interpretation. Cells growing, in 

 culture media outside the body have thrown 

 light upon this question. While fibro- 

 blasts, muscle cells, neuroblasts have often 

 been described as forming syncytia, it has 

 been shown that supposed nerve nets may 

 often be resolved later by separation of 

 the apparently fused processes (Harrison 

 1910; Levi 1917).^ Similar observations 

 have been made on mesenchyme (Lewis 

 1922) and heart muscle (Levi 1925). 



Viewed pragmatically, the experimental 

 analysis of the processes of development is 

 greatly facilitated by considering the em- 

 bryo as made up of discrete units asso- 

 ciated with each other in various degrees 

 of intimacy. 



In relation to development, the most im- 

 portant fact in connection with the cell 

 theory is that each organism begins its 

 independent existence as a single cell, 

 which, barring parthenogenesis, is the 

 product of fusion of two unlike cells, one 

 derived from the body of each parent. 

 This is the narrow bottle-neck through 

 which we have all passed, the book in 

 which all our members were written, when 

 as yet there was none of them. 



Since every organism develops in a cer- 

 tain environment within rather narrow 

 physical limits, its developmental processes 

 can be properly considered only in relation 



3 For the contrary view see Bauer (1932). 



to this environment, as Child has already 

 emphasized. In arranging this symposium 

 Taylor has distinguished between external 

 and internal environment, and has left to 

 me the task of interpreting the meaning 

 of the latter. I shall therefore define the 

 internal environment of a cell as the sum 

 of all those factors with which it is in 

 relation that lie within the organism but 

 outside of the cell in question. These fac- 

 tors affecting the differentiation of the cell 

 change with the successive phases of the 

 development of the organism. 



Strictly speaking, a single cell has only 

 an external environment. However, the 

 germ cells that unite to form the fertilized 

 egg do have in a certain sense an internal 

 environment inside the body of the parent 

 organism. When they are cast loose they 

 cease for a time to have such an environ- 

 ment, and it is only after cleavage begins 

 that the resulting cells acquire a new in- 

 ternal environment in their sister cells. 

 For a short period this is the only internal 

 factor that counts, there being as yet no 

 general milieu interieur. However, during 

 this period, which in the vertebrate embryo 

 includes segmentation, gastrulation and 

 neurulation, the most important differenti- 

 ations occur. Later, after the establish- 

 ment of a circulating medium and the 

 primitive nerve paths, an internal environ- 

 ment of the ordinary sort is again present. 



The ovum, as it grows, acquires a large 

 amount of cytoplasm which enters into the 

 composition of the embryo, though much 

 of the material may be mere inclusions of 

 a purely nutrient nature. So long as the 

 egg is part of the maternal body, its exact 

 relations to the contiguous cells of the 

 parent organism, and, secondarily, to the 

 circulating medium are of prime import- 

 ance in setting up its organization or con- 

 figuration. At this time the egg is either 

 attached to the maternal tissues at one pole 

 or is tightly enclosed in a capsule lined by 

 follicle cells. In many, if not in all cases, 

 but especially in those of the first group, 

 the material is laid down in the egg around 

 a polar axis but with marked gradients 

 along this axis, by which a visible polarity 

 is established in the egg. The nucleus lies 



