MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



425 



lular medium. This view does not commend itself to me. It is opposed to my own observations on similar nuclei 

 in the Spiders. It does not fit in with our knowledge of the nature of the ovnni, and can not be reconciled with thn 

 segmentation of such types as Spiders, or even Enpaguriis, with which the segmentation in insects is undoubtedly 

 closely related. (Com/). Embryology, Vol. i, p. 111). ) 



This discussion seems to have arisen from :i confusion of the morphological ;uid physiological 

 significance of the cell. The segmentation of the nucleus and its surrounding protoplasm is plainly 

 the only important phenomenon, and the segmentation of the yolk is not merely a secondary process, 

 but in many cases a wholly unnecessary one, as we see in the early phases of many .Decapods. 

 In these cases the individuality of the yolk pyramid is temporarily sacrificed or subordinated to 

 that of the true cell, which is surrounded by unsegmented yolk. Later, when the yolk has become 

 divided, the yolk segment or pyramid is gradually reduced until we get the superficial, embryonic 

 cell, with more or less definite boundaries. All the elements of the egg, whether superficial or 

 amoeboid, are clearly to be regarded as cells in a fundamental, physiological sense, as shown by 

 the part which they play in the development of the embryo. 



SECONDARY SEGMENTATION OK THE YOLK. 



There are traces of a secondary segmentation of the yolk in Alpheus during the second, third, 

 , and fourth stages; that is, from the period of invagination to the outlining of the primary append- 

 ages. The yolk spheres arrange themselves in spherical clusters or balls, so characteristic of the 

 early development of nearly all the Arthropods. The yolk ball contains at least one yolk nucleus 

 with perinuclear protoplasm and corresponds to a yolk pyramid, being a cell in the same sense as 

 the latter. Various phases of this secondary segmentation may be seen by glancing over Pis. xxx- 

 xxxv In one egg, which I sectioned just prior to invagiuatiou (Fig. 46), there appears a segmen- 

 tation of the yolk around the central nuclei. 



Bobretzky attributes a morphological value to the secondary segmentation of the yolk in 

 Arthropods, supposing it to be connected with the spreading and final establishment of the ento- 

 blast. The secondary yolk pyramids or giant endoderm cells, which form the lining of the midgut 

 of the embryo crayfish, he compared with the DotterbaUen of Oniscus and Pahtmon. In Palremon 

 the food yolk breaks up into round or polygonal pieces soon after the blastoderm is formed, while 

 in Ouiscus certain cells pass into the yolk from the keel or germinal eminence and gorge them- 

 selves with the yolk substance until they form large balls, which represent the eudoderm (Darm- 

 driisenzelleu). It is stated, however, by Nusbanrn (44) that a part of the eudoderm of Oniscus 

 which gives rise to the gastric gland arises from primitive inesoblast, and in insects the endoderm 

 is formed independently of the yolk cells. The history of the yolk cells and of the wandering cells 

 will be discussed in another section. 



VI. CELL DEGENERATION. 



The rapid and often extensive breaking up and final disappearance of embryonic cells in the 

 course of Arthropod development is a very remarkable phenomenon, and strange to say, it has 

 almost escaped attention up to the present time among the Decapod Crustacea. A study of this 

 subject in Alpheus, Astacus, and Homarus has convinced me that the peculiar bodies described 

 as secondary mesoderm cells in the crayfish (54) correspond to the degenerating, sporelike par- 

 ticles which characterize similar stages in the development of both Alpheus and Ilomarus. 



In some early notes on the development of Alpheus I called these nuclear fragments "spores" 

 (22), but the term is inappropriate if we are dealing with cells in the process of dissolution, as is 

 undoubtedly the case. The anomalous "secondary cells," which have been a sort of outstanding 

 puzzle to embryologists, receive, in my opinion, a more reasonable explanation on the ground that 

 they represent degenerating elements. This view is supported by a comparative study of embry- 

 onic growth in other Arthropods. 



Atyhcus. Degenerating cells are present in Alpheus saulcyi in considerable numbers when the 

 nauplius appendages are budding and increase for a short time beyond this period. They continue 

 in greater or less quantity until six to eight pairs of postoral appendages are formed, when they 

 disappear from the embryo almost completely. They vary in size from small refringent particles 

 to spherical masses as large as ordinary nuclei, or even larger. Many nuclei, instead of having 

 the normal appearance, in which the chroinatiu has the form of a coil or a reticulum with stellate 



