A STUDY UST MORPHOLOGY. 
69 
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. * 
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 
* 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.’ 
