THE AMERICAN LOBSTER. 207 



are in various stages of division, as shown by the clearly defined karyokinetic figures. 

 The surface of the egg at the sides, but particularly in front of this area, presents a 

 striking, and, for a transitory period, a very characteristic appearance. The nuclei 

 are grouped in pairs, in short strings, or in clusters or nests of a dozen or more. In 

 many cases these nuclei are breaking down and giving rise to the "plasma vesicles" 

 and "chromatin nebula'" of Bumpus (30). The degenerating chromatin of these 

 disrupted cells still reacts vigorously upon the staining fluids, and appears as a 

 clouded mass of tine particles (fig. 237, Dg.) which in time becomes more diffused and 

 scattered amid the adjacent yolk granules (fig. 241). The pairs or chains of cells 

 arise by the usual process of indirect division in radial planes. 



The cell nests, as illustrated in fig. 245, plate 52, are the result of a multiple karyo- 

 kinesis, and are formed immediately by either one or two divisions from a single cell 

 (yn x ). Since the active phase of this process lasts but a brief interval, it is not surprising 

 that it usually escapes attention. In this egg, nests of nuclei are very abundant at the 

 sides and immediately behind the region of ingrowth, and occur, as is well shown in 

 fig. 245, both at the immediate surface (en) and below it. In most cases a yolk ball is 

 formed with very definite outlines, its size depending upon the amount of protoplasm 

 which it contains. The yolk ball is strictly analogous to the superficial yolk-bearing 

 cell and to the yolk pyramid. The superficial cell, which is a direct descendant of the 

 enormous yolk segment or pyramid, has this peculiarity in the lobster: By the time 

 a distinct blastodermic envelope is formed it tends to become distinctly separated 

 from the rest of the egg. A definite stratum of cells is thus formed consisting of yolk- 

 laden discoidal or columnar cells. (See figs. 251 and 255, ec.) 



The appearance of the cell nest or cluster in a resting condition is shown in figs. 245 

 and 247. The yolk immediately surrounding it is usually, but not always, segmented 

 into spherical masses. Upon the side of the egg, opposite the embryo nuclei are far 

 less numerous and very uniformly distributed. jSTo cell nests or evidence of active 

 division are seen. 



It will probably be found that, whenever clusters or nests of nuclei appear in 

 the blastoderm or other parts of the embryo of Arthropods, they are the result of 

 multiple cell division. Some time ago I suggested (94, p. 427) that this would account 

 for the nuclear clusters which Reichenbach has figured in the large endodermal cells 

 which form the lining of the mesenteron in Astacus, and which he supposed were due 

 to a process of direct division (163). Both in this case and in the lobster the division 

 is attended by the dissolution of some of the chromatin. 



The histology of the embryo during the invagination period is illustrated by figs. 

 246, 251, 254. At a very early stage a few cells break with the surface and migrate a 

 short way into the egg. A depression about the point of ingrowth soon appears, and 

 the cells, being bathed with nutriment, multiply rapidly until the condition illustrated in 

 fig. 251 is reached. They here form at the surface a definite layer of prismatic elements, 

 each containing a quantity of yolk with definite boundaries. It should be noticed also 

 that the nuclei of the in-wandering cells are often inclosed in spherical masses of yolk. 



The histological processes which occur at this period vary considerably in different 

 embryos. Thus in fig. 240 we see a stage of development very similar to that of 

 fig. 252, but a little earlier. In the former (fig. 240) the cells about the area of 

 invagination have multiplied until they form a large cluster at the bottom of the pit. 

 A syncytium is formed, and the protoplasm of the outermost cells lies at the surface, 

 while the neighboring yolk is thrown into long, tapering segments. Some of these 



