NA TURE 



511 



THURSDAY, OCTOBER 28, 1909. 



THE CLEAVAGE OF THE OVUM. 



Lchrhiich der vcrgleichenden Enhuickhingsgcschichte 

 der wirhellosen Thiere. AUgemeiner Theil, Dritte 

 Lieferung. By Prof. E. Korschelt and Prof. K. 

 Hcider. Pp. 166. (Jena : Gustav Fischer, 1909.) 

 Price 4.50 marks. 



IN tlie development of the individual organism 

 there are involved growth, cell-division, and dif- 

 ferentiation. It is with the second of these that 

 the present instalment of a splendid text-book deals. 



Apart from certain alterations of egg-structure 

 which are consequent upon fertilisation, the sub- 

 division of its material is the first sign the fertilised 

 ovum gives of its activity, and this " segmentation " 

 proceeds with great regularity until other events — 

 germ-layer formation — supervene. The problems 

 that at once arise are therefore : (i) What is it which 

 sets a term, not, indeed, to cell-division, for that 

 continues, but to the period of segmentation ? (2) To 

 what causes must the definite pattern of division — 

 spiral, radial, or bilateral, and so on — be attributed? 

 and (3) Is there ariy necessary relation between the 

 manner in which the ovum is thus cut up and the 

 ultimate processes of differentiation? 



In this volume — a very admirably written and 

 beautifully illustrated description of the various types 

 of cleavage — these problems are to some extent 

 touched upon, and we are glad to see that the 

 view is taken that it is the attainment of a definite 

 size-relation between nucleus and cytoplasm which 

 defines the end of this, the first phase of develop- 

 ment (p. 7), though Boveri's important experiments 

 are not noticed here. 



In respect of the other two problems — which, 

 indeed, largely overlap — the position which the 

 authors appear to have adopted— that cleavage is 

 necessarilv a process of differentiation, that in 

 some, e.^. in the spiral type, it is a " Mosaik- 

 arbeit " in Roux's sense, and that the blastomeres 

 in this case are not interchangeable (pp. 15, 115)— 

 is not, we believe, wholly justified by the results 

 of experiment, though in dealing with the amphibian 

 egg they admit (p. 38) that the relation between cell- 

 division and differentiation is " wenig fixirt." 



The truth is that Roux's attempt to demonstrate 

 an absolute coincidence of the first furrow and the 

 sagittal plane in the frog's egg, and the consequent 

 formulation of the " Mosaik-theorie," with its 

 imaginary qualitative division of the nucleus, still 

 casts its shadow upon this discussion. That hypo- 

 thesis, however, so far as the nucleus is concerned, 

 has now been abandoned, while recent work has 

 shown that the factors which determine the pattern 

 of cleavage are distinct from the causes of differen- 

 tiation, even in those cases in which cell-division does 

 look like a " mosaic." Thus there is in the frog's egg 

 a far closer relation between the plane of symmetry and 

 the sagittal plane than between the latter and the first 

 furrow. The type of segmentation may be altered, 

 by pressure or other means, without jirejudice to 

 NO. 2087, VOL. 81] 



normal development. In the Ctenophora abstraction 

 of the vegetative egg-substance involves absence of 

 costas in the embryo, but division is still normal, 

 while the isolated D. hJastomere of a Dentalium ovum 

 will produce a normal larva, in spite of the fact that 

 its cleavage is partial, because it contains the polar 

 lobe. Moreover, cells occupying identical positions 

 in similar cleavage systems have not always the same 

 fate. 



The two sets of factors may coincide, but they do 

 not necessarily do so. The causes of cell-division 

 will be found in the relations between nucleus and 

 cytoplasm, surface tensions, and so on, of differentia- 

 tion in the various substances present in the cyto- 

 plasm (and the nucleus), but the way in which this 

 heterogeneous material is cut up in cleavage is a 

 matter of indifference; what is essential is that it 

 should be diminished by division until the necessary 

 ratio between cytoplasm and nucleus has been 

 reached. 



.4 CANADIAN MINING AND METALLURGICAL 

 DIRECTORY. 



Report on the Mining and Metallurgical Industries 

 of Canada, 1907-8. Pp. xvi + 972. Canada, Dept. 

 of Mines, Mines Branch. (Ottawa : Government 

 Printing Bureau, 1908.) Price i dollar. 



THE Dominion of Canada, having the area and 

 varied geological and geographical structure 

 of half a continent, has naturally a great diversity 

 in its mineral wealth. The mining fields are widely 

 scattered, and most of them are imperfectly known. 

 Hence, in accordance with a wise provision of the 

 Canadian Mining Act of 1907, a detailed guide to 

 the mining and metallurgical industries of Canada 

 has been prepared by the Department of Mines. 

 The work has been prepared under the superinten- 

 dence of the director, Dr. Haanel, and edited by Mr. 

 S. Groves. In order to secure the most trustworthy and 

 recent information, a series of special investigators 

 have visited all the mining districts of the Dominion, 

 and numerous short reports on the geology and on 

 mining and metallurgical methods are included from 

 members of the Geological Survey and from mining 

 engineers. The work includes 936 pages of text, 

 and is illustrated by numerous maps and plates, 

 showing the distribution of the mining fields and 

 views of the surface operations at the mines and 

 metallurgical works. 



The book is divided into two main sections, the 

 first dealing with the metalliferous ores and fuels, 

 and the second with building materials, cements, 

 clays, ochre, &c. Each part is subdivided geo- 

 graphically; the history of mining in each province 

 is recorded, and there is a brief, often, perhaps, too 

 brief, statement of the structure of the mining field. 

 The bulk of the work consists of an account of the 

 present condition, including capital, staff, area, 

 equipment, and operations, of every important mine 

 and metallurgical establishment in the Dominion. 

 This valuable directory of Canadian mining gives a 

 clear idea of its growing importance and variety. 



T 



