486 BE VELOPMENT 



of mitosis follows a path somewhat different in the germ cells from 

 that of the somatic cells. The primordial germ cell, after dividing 

 in the same manner, as we have seen somatic cells do, undergoes 

 a process of maturation which is an additional series of divisions 

 resulting in cells with only half the number of chromosomes that 

 the parent cell possessed (Figs. 99 and 100). 



Fertilisation. This is the union of a male cell and a female cell, 

 whereby incidentally the full number of chromosomes is restored 

 and a further series of cell divisions instituted. 



The unfertilised ovum is a moribund body which disintegrates 

 more or less rapidly. If, before disintegrative processes have 

 become apparent, the egg undergoes fertilisation, destruction is 

 stayed. The fertilised egg develops, grows and becomes differen- 

 tiated into various structures. This process of differentiation 

 of protoplasm is an orderly one, taking place always in the same 

 manner and being modified always by the same conditions. 



On the entry of the spermatozoon, some change in the free 

 energy of the egg must take place. The egg is no longer static 

 but becomes endowed with dynamic force. In order to discover 

 the underlying physico-chemical change, Loeb attempted to 

 induce developinent of unfertilised eggs l^y alteration of the 

 environmental conditions. No change in a system in equilibriimi 

 can take place unless the relative amount or incidence of the free 

 energy of the environment is first altered. The two series of 

 changes — external and internal — are cause and effect. This is 

 merely a restatement of the Law of Inertia. The entry of the 

 spermatozoon alters the balance of free energy between egg and 

 environment. Loeb attempted to bring about the same result 

 by altering the free energy balance between environment and egg. 

 He found that two separate and distinct changes took place after 

 fertilisation, viz., membrane formation and development. These 

 involve totally different physico-chemical reactions. Membrane 

 formation is not followed necessarily by development. 



I. Membrane Formation. 



Loeb foimd that all those substances or agencies which can 

 bring about haemolysis (Chap. XXII., p. 318) also induce mem- 

 brane formation. The best agent for this purpose is dilute butyric 

 acid. Immersion of sea-urchins' eggs (unfertilised) in sea-water 

 containing about 5 per cent, of A'^/10 butyric acid for 2 to 4 minutes 

 })rings about typical membrane formation. This membrane is 

 tough, and is separated from the egg-substance by a layer of more 

 fluid material. 



Examination of the performance of the other cytolytic sub- 



