CELLULAR DIFFERENTIATION AND INTERNAL ENVIRONMENT 



81 



the ovarian egg and the later arrangement 

 assumed after maturation and fertilization 

 show regional differences, which must be 

 the result of a sorting out process. This 

 requires energy and, according to some, 

 even intelligence. It may be related in 

 some way to the bioelectric potentials of 

 both the ovum itself and the surrounding 

 maternal tissues. 



In the orientation of the protein mole- 

 cules of the egg cell, due to their dipole 

 character, a paracrystalline configuration 

 is set up, and large numbers of molecules 

 of water and electrolytes enter into definite 

 relation with it. There are also associated 

 carbohydrates and lipoids. Some of the 

 material is built up into the living proto- 

 plasm, and the composition of this proto- 

 plasm shows regional differences corres- 

 ponding to the main axis of the egg. The 

 ground cytoplasm of the egg varies along 

 this axis, retaining throughout, however, 

 the configuration characteristic of the 

 species. In addition to the material that 

 becomes a part of the ground cytoplasm 

 there is also other material which forms 

 aggregates that do not enter into the com- 

 position of the cytoplasm so intimately but 

 take on granular or vesicular form and 

 constitute visible inclusions, such as pig- 

 ment granules, oil droplets, mitochondria, 

 and yolk. 



The inclusions are also re-distributed in 

 a definite way after fertilization and since 

 they are parcelled out to particular cells 

 which give rise to particular organs, they 

 have often been termed organ-forming sub- 

 stances. Experiments in centrifuging eggs 

 show, however, that they usually play no 

 such role, for they may be distributed 

 very abnormally by this treatment without 

 producing abnormalities in the resulting 

 embryo. 



In accordance with the foregoing, the 

 polarity of the egg may be based upon 

 three factors, the dipole character of the 

 protein molecules and their resultant ori- 

 entation, regional gradational differences in 

 the composition of these molecules, and 

 regional differences due to cytoplasmic in- 

 clusions. The first is a polarity of orien- 



tation (Richtungspolaritdt) , the third a 

 polarity of stratification {Schichtungspo- 

 laritdt) (Boveri 1901; Driesch 1908; Przi- 

 bram* 1913), while the second is a polarity 

 in which probably both orientation and 

 stratification are concerned. The position 

 of the nucleus in the cytoplasm also has a 

 definite relation to the polarity of the egg. 



According to the hypothesis here adopted, 

 the polarity of the egg, and hence that 

 of the resulting organism, is referable pri- 

 marily to the orientation of large mole- 

 cules, the arrangement of which is deter- 

 mined in relation to their internal environ- 

 ment within the maternal body as well as 

 by their mutual relations. The asymmetry 

 of the organism may likewise be assumed 

 to follow ultimately from the asymmetric 

 configuration of the molecules. "While these 

 relations are normally fixed, they may, 

 however, be inverted by external factors, 

 as in the production of situs inversus 

 (Spemann 1906; Pressler 1911) or in the 

 inversion of the polarity of the Fucus egg 

 by means of directed light and other phys- 

 ical factors (Whitaker 1931, 1936). 



While the correlatives of the form and 

 structure of the adult organism are to be 

 sought in the molecular characteristics of 

 the substances composing the egg — and 

 these must be considered as persisting 

 throughout all stages of development — a 

 more definite macroscopic topographic re- 

 lation between egg and organism than ex- 

 isted before is entered into after the 

 streaming that follows maturation and 

 fertilization comes to repose. To what ex- 

 tent this secondary arrangement is corre- 

 lated with the differentiation of the em- 

 bryo is a question to which many have 

 addressed themselves. There is much 

 variation in the answer according to the 

 species of egg studied. 



Rapid changes take place in the egg cyto- 

 plasm during maturation and fertilization 

 and immediately thereafter. If fragments 

 are removed from the egg in the early part 

 of this period, the embryo may develop 

 without defect except in size, especially 

 when the egg is cut vertically, i.e., parallel 



4 Op. cit., p. 35. 



