66 PKOCEEDXNGS OF THE MAXACOLOGICAL SOCIETY. 



Mucli of the experimental work already achieved in Biology has 

 lain with the egg, and with the early stages of development, at which 

 susceptibility to disturbing influences and the action of external 

 stimuli is greatest ; but, side by side with progress in this new 

 departure, there has come about a more exact determination of the 

 detailed processes at work during normal development. Our faith in 

 the germ-layer theory, at any rate so far as it involves the so-called 

 "mesoderm," is shaken to its foundations. We now know that what 

 are usually termed homologous parts may differ fundaaientally in 

 origin under different conditions of development, and the structure 

 and relations of the developed organ become once more the standards 

 of comparison.^ Following in the wake of Whitman and his classical 

 monograph on the development of the leech Clepsine, we now seek 

 to determine the relative values of the individual embryo-cells by 

 their ultimate fate. " Cell-lineage " is the term applied to this 

 most important departure in Comparative Embryology, and in the 

 hands of Wilson, Kofoid, Castle, and others, it is revolutionizing 

 our conceptions of the fundamentals of Embryology. As Kofoid's 

 investigation^ cliiefly concerns a common slug {J f/ noli max agrestis), 

 a word in detail concerning it. In the first place, having observed 

 an alternation in the direction of the spindles and the planes 

 of division of the blastomeres, he formulates a law of " spiral 

 cleavage," as distinct from "radial" and "bilateral." Critically 

 examining the work of his predecessors, he proceeds to show that in 

 a number of other Mollusca evidence for the operation of this spiral 

 cleavage has been obscured by the nomenclature employed. He, 

 however, formulates one "law," but to challenge another, and that 

 bearing the honoured name of Francis Maitland Balfour. It was one 

 of Balfour's greatest achievements to have attempted to correlate 

 the rate of segmentation and the size of the blastomeres with the pro- 

 portional development of food-yolk. " Where the yolk spherules 

 are fewest," he wrote, " the active protoplasm is necessarily most 

 concentrated, and we can lay it down as a general law that the 

 velocity of the segmentation in any part of the ovum is, roughly 

 speaking, proportional to the concentration of the protoplasm there ; 

 and that the size of the segments is inversely proportionate to the 

 concentration of the protoplasm." Since Balfour's time it has 

 become evident that of two eggs otherwise similar, the one bearing 

 tlic larger amount of yolk cleaves the less rapidly, and that, all things 

 considered, we may regard the greater development of food-yolk as 

 favourable to the reduction or absence of metamorphosis, and the more 

 rapid assumption of the fully-developed state. In Agriolimax and 

 Umhraculum {UmhreUa, auct.), however, on Kofoid's showing, it is 

 the larger of the two first-formed cells which is the first to divide, 



1 Cf. E. B. Wilson, " Embryological Criteriou of Homology": Wood's Holl 

 Lect. for 1894, Boston, 1895, p. 101. 



^ Bull. Mus. Corap. Zool. Hiirvard, vol. xxvii, No. 2 ; and prelim, paper in Proc. 

 Amer. Acad. Arts and Sci., vol. xxix, p. 180. 



