436 SUMMARY OF GURliENT KESEAKCHES RELATING TO 



pig lasts sixteen to eighteen days, and that the mammary gland shows 

 two phases in this cycle. The first, depending upon the absence of the 

 corpus luteum and the activity of another constituent of the ovaries, 

 is marked by proliferation. In the second, proliferation is as a rule 

 absent, though it may occur towards the end of the phase ; it is probably 

 due to substances secreted by the corpus luteum. 



Can Artificially-activated Eggs be Fertilized ? * — C. R. Moore has 

 experimented with eggs where artificial activation has been complete, as 

 indicated by membrane formation and the absence of Lillie's fertilizin. 

 In such cases the superposition of fertilization is impossible. Where the 

 activation has been only partial, a partial fertilization may be effected, 

 but development in such a case is not normal. 



Growth and Form.t — D'Arcy Wentworth Thompson has made an 

 important contribution to Bio-physics, and his book touches at many 

 points the interests of the microscopist. It is an application of some 

 of the concepts of physical science and sundry mathematical methods 

 to the study of organic form. A beginning is made with Galileo's 

 " principle of similitude," that " neither can man build a house nor can 

 Nature construct an animal beyond a certain size, while retaining the 

 same proportions and employing the same materials as sufficed in the 

 case of a smaller structure." Differences in degree of magnitude are 

 associated with profound differences of physical property and potentiality. 



The rate of growth is the sub^'ect of a long discussion, for the form 

 of an organism is usually a direct expression of a rate of growth which 

 varies according to its different directions. The velocities in different 

 directions tend to maintain a ratio which is more or less constant for 

 each specific organism ; and to this regularity is due the fact that the 

 form of the organism is in general regular. 



There is a long and fascinating discussion of the bio-physics of the 

 cell, which is treated as a sphere of action of certain more or less 

 localized forces, of which surface-tension is the one especially responsible 

 for giving the cell its outline and its morphological individuality. 

 The author seeks to show that cell-division and other intra-cellular 

 phenomena may be tentatively explained as the results of a conflict 

 between surface-tension and its opposing forces. The phenomena of 

 karyokinesis are analagous to, if not identical with, those of a bipolar 

 electrical field. The forms of free cells are essentially dependent on 

 surface-tension, and that along with the laws of equilibrium and the 

 principle of minimal areas may be said to go far in interpreting the 

 forms of cells in aggregates or tissues. 



It is pointed out that all possible groupings or arrangements what- 

 soever of eight cells, none being submerged or wholly enveloped, are 

 referable to one or other of thirteen types or forms, or perhaps fewer, 

 since some are unstable. 



A very interesting chapter discusses concretions, spicules, and spicular 



* Biological Bulletin, xxxi. (1916) pp. 137-80. 



t On Growth and Form. Cambridge : 1917, xv and 793 pp. 



