706 JOHN BEARD, 



The question of the relationship, which may be supposed to subsist 

 between the embryo — or for the matter of that, the mature organism 

 — and the germ-cells within it, is one of the deepest and most im- 

 portant in the range of embryology. In the opinion of most embryo- 

 logists the organism forms the germ-cells, and anon, when these are 

 fertilised, they give rise directly to new organisms, which in their 

 turns at the proper time produce from the tissues of their bodies 

 new sets of germ-cells! 



The organism neither forms the germ-cells, nor is it the chief, 

 nor even the immediate, task of any one of these to give rise to a 

 new organism ! These statements cannot be proved here, nor is this 

 the proper place for the production of the evidences, afforded by the 

 study of the actual development. Briefly, after fertilisation the egg 

 segments or cleaves itself a certain number of times. In the embryology 

 of text-books the whole of the early cleavage products are regarded as 

 the bricks, out of which the embryo is gradually built up. Thus, in an 

 egg, which has divided five times, the number of segments will be at the 

 most 2, 4, 8, 16 and 32, these 32 cells being generally considered to re- 

 present the beginnings of the future embryo. This period of 32 cells 

 is taken for a special reason. In certain cases by the time the fifth 

 cleavage is over, and 32 cells are present, there is no trace whatever 

 of the future embryo. This statement may be made still stronger by 

 the assertion, that at this period not even the commencement at the 

 formation of an embryo has been set about. Of the 32 cells then 

 present 31 are usually transitory or larval in nature, being destined 

 to form the asexual organism, the larva or phorozoon, upon which 

 the embryo arises. The remaining cell is, or becomes, the primitive 

 germ-cell (= U. K. Z, the "Urkeimzelle" of German authors). In a way, 

 which cannot be described here, this primitive germ-cell has a direct 

 ancestry along a straight line from the fertilised egg. It now divides 

 a certain number of times, this number varying in different species. 

 Apparently, in the common frog it divides 3 times, in the lamprey 

 5 times, in the dogfish, Scyllium canicula, 7 times, in the develop- 

 ment of the male skate 8 times, and in that of the female 9 times. 

 In this way the number of the products of the primitive germ-cell 

 comes to be one of the following: 2, 4, 8, 16, 32, 64, 128, 256, 



b. The germ-cells of Pristiurus, in: Anat. Anz., 1902, 



7. The numerical law of the germ-cells, ibid. 1902. 



8. The Germ-Cells, Pt. I, in: Zool. Jahrb., V. 16, Anat., p. 615 

 —702, 1902. 



