64 PRINCIPLES OF EMBRYOLOGY 



in a manner which closely simulates the normal cleavage pattern, and 

 that there are cycles of activity of the nucleus, involving the appearance of 

 asters, usually monocentric but occasionally leading to true mitoses. 

 Although the lobulations eventually disappear, so that the egg regains its 

 spherical shape, their occurrence is important evidence that autonomous 

 cytoplasmic (probably cortical) changes play a part in the normal cleavage 

 process (see also Lehmann 1948(7). 



The 'differentiation' performed by these eggs is also by no means com- 

 plete. The most they accomphsh is the separation into different regions 

 of various types of cytoplasm, together with a relative movement of the 

 outermost clear cytoplasm over the vegetative material which somewhat 

 recalls the normal process of gastrulation, and finally a differentiation of 

 cilia; but there is no true organogenesis, such as the formation of a gut or 

 apical tuft. Nevertheless, the evidence that even this segregation of 

 ooplasmic components can proceed as far as it does when the egg is not 

 divided up into cells, indicates the importance of processes which go on 

 within the body as a whole. It becomes clear that it is dangerous to 

 attribute too much importance to the cell as the basic unit on which 

 everything depends, and that theories such as that of Weiss (p. 413) which 

 attempt to explain development in terms of the properties of cell mem- 

 branes can be at best a part of the truth. 



4. Cleavage without nuclei 



, It has been mentioned in the last chapter that parthenogenetically 

 activated eggs or egg fragments may undergo fairly regular cleavages 

 even in the absence of nuclei. Cases of this have been described by Harvey 

 (1936, 1940) in echinoderms, by Gross (1936) in Artemia, and Fankhauser 

 (1934) and Briggs, Green and King (195 1) in Amphibia, Some authors 

 question whether the phenomenon is truly comparable to cleavage, and 

 suggest that it is more like the disorganised 'bubbling' that some cells 

 undergo at the time of division, but in the best cases described in the 

 Amphibia something very like a normal blastula is produced, and it 

 seems unduly sceptical to deny the process the name of cleavage (Fig. 4.3). 



5. The mechanism of cleavage 



Cleaving eggs, particularly those of marine invertebrates, have fre- 

 quently been used to study the general problems of cell division. This is 

 an enormous subject, involving the behaviour of the chromosomes, of 

 the achromatic apparatus (spindle, centrosomes, asters, etc.) and of the 

 body of the cell which becomes divided in two. A full discussion of all 

 aspects of it would lead us too far into the fields of cytology and cell 



