202 ESSENTIALS OF BIOLOGY 



when the differentiation of the cytoplasm had not occurred and 

 the egg is then isotropic, or of equal cytoplasmic constitution 

 in all parts. This is the state of the sea-urchin ovum, and also 

 of the 8-, 1 6- cell, and blastular larvae. There is, as yet, no 

 differentiation of the parts and the latter are still equipotential 

 so that they can effect regulations. Some time, however, in the 

 development this equipotentiality of the parts is lost. 



Clearly the gonidial cell, before it matures, the matured and 

 ripening ovum and the embryo in process of developing are 

 phases in a life-history and at any such phase the thing that is 

 developing is an organism. It takes up nutritive materials from 

 its environment, chemically transforms these and assimilates them 

 so as to form new cells. It assembles these cells as they come 

 into existence in specific configurations that are the anlagen, or 

 rudiments of organs, and it proceeds doing this, more or less 

 rapidly perhaps, in spite of quite notable changes in the physical 

 conditions of the development. If these conditions are violently 

 changed the course of organogeny, or of the subsequent larval 

 metamorphoses may be changed, but very often the embryo, of 

 itself, can regulate its activities so that the normal development 

 is effected. If the environmental changes are too violent a 

 malformed embryo, or larva is, of course, formed. The develop- 

 ing organism may inhabit a highly specialized environment — as 

 when it is in utero and is nourished by the maternal blood, or when 

 it is part of an egg containing all the nutritive materials necessary 

 for development, but these conditions are no more specialized 

 than are those in which many adult, parasitic organisms live. On 

 the other hand the ova, embryos and larvae of molluscs, worms 

 and hosts of invertebrate animals live in the open sea, obtain 

 their nutriment from the substances in solution therein and 

 behave in all ways as if they were independent organisms — except 

 that their development is incomplete. 



We now proceed to discuss the nature of the developmental 

 process and first it is necessary to examine into the problems 

 presented by the cell nucleus. 



73. ON THE CELL NUCLEUS IN DEVELOPMENT 



Typically the cell is a minute rounded body containing a 

 nucleus. Apart from the latter the cell-substance or cytoplasm, 



