74 PRINCIPLES OF EMBRYOLOGY 



Raven (1948) has estimated the 'tension at the surface' of the eggs of 

 Limnea from fertihsation till first cleavage. His method w^as to centrifuge 

 the egg for 5 minutes at a moderate speed, which gave a force of i86og. 

 Observation of the distinctness with which the internal contents become 

 stratified gives an indication of the viscosity of the cytoplasm, while the 

 extent of the elongation of the egg allows one to draw some tentative 

 conclusions about the deformability of the surface. The elongation will, 

 however, also be affected by the degree to which the internal con- 

 stituents separate into defmite layers, so that the viscosity changes some- 

 what obscure the picture of the alterations in the properties of the surface. 

 Raven finds that the viscosity is low immediately before each of the 

 first three cleavages, but rises as the furrow makes its appearance, and 

 reaches a maximum about 10-15 minutes later. The data on the surface 

 tension are not so clear cut, and only the first cleavage has been investi- 

 gated. Raven finds that it is low immediately before the appearance of 

 the furrow. His diagrams show, although he does not refer to the fact 

 in discussing them, that it rises steeply during the cleavage. It is not quite 

 clear whether this rise occurs at a slightly later period than the similar 

 increase in stiffness of the cortex of the echinoderm egg, but, apart from 

 possible minor changes in timing, the phenomena seem rather alike in 

 the two groups. 



These results are good evidence that some, at least, of the processes 

 envisaged in Swann and Mitchison's theory actually occur. Moreover 

 there is plenty of other evidence that expansions of the cell membrane 

 are often associated with cleavage. Some of the most extreme examples 

 of this are seen in those spirally cleaving eggs in which a polar lobe is 

 formed (Fig. 6.2). But the mere occurrence of an expansion does not 

 suffice to show that it is the prime mover in bringing about a cleavage of 

 the cell, and there is some reason to doubt whether it is more than one 

 out of a number of factors which may play a part. For instance, we have 

 seen that in Tubifex there are considerable cortical expansions which 

 cause the egg to throw out protuberances at the times of polar-body 

 division and at the first cleavage division. The second cleavage division 

 occurs first in CD blastomere and only somewhat later in the AB one, 

 and is not accompanied by such a pronounced cortical expansion as the 

 earlier ones, though the CD cell does elongate considerably and increase its 

 surface area to a fair extent just before the cleavage occurs. Huber (1947) 

 has studied the action of two anti-mitotic substances, naphthoquinone 

 and phenanthrenequinone. He finds that the former has a particularly 

 strong effect on the expansions, tending to suppress the formation of 

 protuberances in the first division and the stretcliing of the CD cell in the 



