102 PRINCIPLES OF EMBRYOLOGY 



as 'mitochondria'. These differences, which it must be remembered 

 affect only the concentration of the particles, none of which are com- 

 pletely absent in any region, cannot be seen in the newly fertilised egg, 

 which contains a population of microsomes which appears uniform in 

 the electron microscope. It seems most probable, however, that these 

 microsomes differ in their chemical properties, and that it is the micro- 

 some population which determines the character of the ooplasms. 



The second problem which has been attracting attention recently is 

 that of the mechanisms of localisation. The original distribution of mater- 

 ials in the egg can be fairly easily disturbed by centrifugation, as a conse- 

 quence of which the egg contents become stratified into layers of 

 different specific gravity. As would be expected if the eggs behaved in a 

 strictly mosaic manner, this stratification frequently leads to the pro- 

 duction of abnormal embryos. But this is not always the case; almost per- 

 fectly normal larvae may develop from eggs which have suffered a 

 severe stratification. It used to be thought that the explanation of this must 

 be that the substances which become stratified are not those which are 

 morphogenetically active, but comprise only relatively neutral materials 

 such as yolk. However, the application of histochemical tests has demon- 

 strated the presence of several important enzymes in the stratified layers, 

 and it therefore becomes rather unplausible to advance this hypothesis. 

 Raven (1948) has made a particularly careful study of the phenomena in 

 the snail Liumea. He showed that in the unfertihsed egg there is a visibly 

 differentiated 'sub-cortical plasm' located near the vegetative pole, and 

 that soon after fertilisation an 'animal plasm' appears near the animal 

 pole. These two plasms move in defmite ways during the early stages of 

 development; before the first cleavage, the sub-cortical plasm spreads 

 upwards so as to clothe the entire surface of the egg just below the cortex, 

 while the animal plasm also extends somewhat, but eventually comes to lie 

 mainly in the micromeres (Fig. 3.3, p. 49). If an egg is centrifuged in such 

 an orientation that the pole plasms are moved away from their normal 

 location, it is found that they very soon make their appearance again in 

 their original positions. The rapidity of this distribution differs according 

 to the exact stage when the centrifuging is carried out, since the viscosity 

 of the cytoplasm varies throughout the progress of the cleavage. Raven 

 claims that the redistribution of most substances can continue, even after 

 the appearance of the first few cleavage faces; only the protein yolk 

 appears to be relatively immobile and unable to pass through the cell 

 walls. 



There must therefore be some general condition which controls the 

 disposition of the various regions of egg cytoplasm. Both Raven, and 



