80 



THE CELL AND PROTOPLASM 



on this axis but usually excentrically 

 placed with reference to the egg as a whole. 

 Substances emanating from the nucleus 

 may therefore act vectorially on material 

 entering the egg from without. 



At maturation, when a large amount of 

 fluid is discharged from the nucleus into 

 the cytoplasm, and accompanying or fol- 

 lowing the entry of the sperm, a major 

 commotion occurs, during which there is a 

 lively streaming of cytoplasmic materials, 

 resulting in their partial rearrangement. 

 After this many eggs become visibly bi- 

 laterally symmetrical, while others show a 

 clear asymmetry. It is at this period that 

 the egg loses its original internal environ- 

 ment and is discharged either to the ex- 

 terior or to the oviduct of the maternal 

 parent, where a greater or lesser part of its 

 development may take place. 



The polar differentiation acquired while 

 the egg was part of the maternal organism 

 may last in part through maturation and 

 fertilization in spite of the streaming and 

 rearrangement that occurs then. After 

 fertilization the new arrangement of ma- 

 terials follows a certain pattern peculiar 

 to the species, as is well exemplified in the 

 eggs of ctenophores, molluscs, annelids, 

 and ascidians, though less definitely marked 

 in many other forms. When cleavage oc- 

 curs the resulting cells are thus already to 

 a certain extent differentiated from one an- 

 other, as measured by what they do in form- 

 ing the primitive organs of the embryo. 



Little is known with regard to how these 

 differentiations come about, but it may be 

 assumed that they are related in some way 

 to the physico-chemical properties of the 

 material entering into the composition of 

 the egg and to the direction from which 

 they enter it. Of first importance in this 

 connection are the protein molecules, which 

 to the best of our knowledge are polarized 

 and asymmetric. They would thus tend to 

 orient themselves within the cell as they 

 increase. Other substances, either com- 

 bined with the proteins or separate, would 

 tend to concentrate near one or the other 

 pole of egg according to their electrical 

 charge. 



While the asymmetry of protein mole- 

 cules does not preclude them from combin- 

 ing to form crystals of high degree of 

 symmetry, as in the case of pepsin, it may 

 be assumed that, in some way related to 

 this molecular asymmetry, there arises 

 sooner or later in the development of the 

 embryo a macroscopic asymmetry, which 

 characterizes most if not all organisms and 

 is merely masked by a superficial bilateral 

 or radial symmetry. 



Superposed upon the polar arrangement 

 there are also differences between cortex 

 and deeper portions of the egg cytoplasm. 

 This, too, may be due primarily to molec- 

 ular orientation at the surface. In this 

 cortical or ectoplasmic layer, which is 

 usually free from inclusions, we probably 

 have the characteristic protoplasm of the 

 species in its purest form. The behavior 

 of this material in early stages of develop- 

 ment, i.e., during maturation and segmen- 

 tation, has been followed by many ob- 

 servers in a great variety of eggs, since 

 G. F. Andrews (1897) first described the 

 amoeboid spinning movements in the cor- 

 tical layer of starfish and sea urchin eggs. 

 In the eggs of Bero'e a very peculiar 

 streaming of this layer takes place during 

 cleavage (Spek 1926). Everything points 

 to the importance of this cytoplasmic con- 

 stituent in the early processes of develop- 

 ment (Just 1939). 



The use of the term protein molecule in 

 the foregoing requires some further ex- 

 planation. As stated by Chambers, the 

 proteins extracted from cells or those in 

 dead cells are different from living proto- 

 plasm and are doubtless much less complex 

 in their configuration. Such arrangements 

 as occur in the living are upset at death 

 or by the chemical processes used in sepa- 

 rating the constituents known as pure pro- 

 teins. These changes are, so far as we 

 know at present, irreversible, so that study 

 of the properties of pure proteins alone 

 can never give an adequate picture of liv- 

 ing protoplasm, though knowledge of these 

 substances is essential to an understanding 

 of it. 



Both the arrangement of materials in 



