A STUDY IN MORPHOLOGY. 09 



of energy which, at length, becomes sufficient to overcome this resistance, and to 

 initiate a period of activity which lasts until the whole of this reserve of force has been 

 expended in the rearrangement of the protoplasm. The physical properties of the 

 protoplasm now reassert themselves, and tend to reduce the whole egg as nearly as 

 possible to a spherical form once more, and the egg then remains inactive until the 

 supply of energy again becomes great enough to overcome the resistance. 



If this is the true explanation we should expect to find the alternation of rest and 

 activity much more general than the change of form, for the degree of consistency of 

 the protoplasm or the amount or character of the food-material, or the way in which 

 it is distributed through the egg, may prevent the second set of changes from showing 

 themselves. This is precisely what we do find, and in the bony fishes, where the large 

 food-yolk would prevent any marked change of form, we find the first set of changes 

 well marked, but with no trace of the second set. 



Leaving this subject for the present, I wish to say a few words about another 

 interesting phase of the early stages of Lucifer. We cannot fail to be impressed by 

 the very remarkable departure from ordinary Arthropod segmentation, nor can we 

 overlook the fact that in all the points of difference from the eggs of allied forms, the 

 eggs of Lucifer show a most suggestive resemblance to the ordinary unspecialized ova 

 of other Metazoa. 



In an ordinary Arthropodan egg we have, as the outcome of the process of segmen- 

 tation, a central mass of food-yolk, which may or may not be divided into segmentation 

 products, and which completely fills the segmentation cavity ; and an outer investing 

 layer of blastoderm cells; that is, the egg undergoes a centrolycethal segmentation." 5 ' 



In most Crustacea the early stages of segmentation are regular, and apparently 

 total, but the lines of cleavage do not pass entirely through the egg, and the spherules 

 are united to each other by a central mass of food-yolk. When segmentation is 

 somewhat advanced the products of segmentation become more or less pyramidal, 

 with the bases of the pyramids at the surface, and their apices fused together at the 

 centre of the egg. The outer ends of the pyramids then become transparent and 

 separate off as a blastoderm, while the inner portions usually fuse together, more or 

 less perfectly, to form a central food-yolk, which fills the space which in ordinary eggs 

 constitutes the segmentation cavity. A small portion of the blastoderm then becomes 

 invaginated to form the primitive digestion cavity, and the remainder becomes the 

 ectoderm. 



The centrolycethal type of segmentation presents great variations in the different 

 groups of Arthropods, but in nearly all cases its peculiarities are so well marked 



f The whole subject of segmentation has been so ably and exhaustively reviewed by BALFOUR in his 

 recent work on ' Comparative Embryology,' that it does not seem necessary to burden this paper with 

 a long list of references to the literature of Arthropod segmentation, or to enter into an exposition of the 

 present state of our knowledge of the subject. All the essential facts and opinions may be found on 

 pages 79-99, 317-379, and 425-433 of vol. i. of the ' Comparative Embryology.' 



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