THE AMERICAN LOBSTER. 213 



generally underlie the triangular and U-shaped areas" and are "found as small clouds 

 between and also outside the limits of the embryonal tract." He says further that 

 "chromatin grains, as was seen in the surface view, are mosc abundant where ecto- 

 dermal cells are most numerous." No inference is drawn from this, but plainly the 

 true one is not that the larger collection of endoderm cells are centers of cell degener- 

 ation, but that the mesoblastic cells attach themselves to those parts of the embryonic 

 ectoblast which are growing the fastest, and by their own dissolution give rise to the 

 "chromatin nebulae." 



Bumpus does not explain the origin and fate of the chromatin particles which he 

 accurately figures, but remarks that " structures comparable with the chromatin grains 

 of the plasma cells are neither mentioned nor figured by Reichenbach, though the 

 so-called 'serum' may represent the region of their activity." Further on it is said 

 that "future comparison may prove these," the " secondary mesoderm cells," of Eeich- 

 enbach "to be the same as the plasma vacuoles and their chromatin grains;" and 

 again, "I have been unable to find in Homarus preparations that throw any direct 

 light on the so-called 'secondary mesoderm.'" 



I have shown (94) that the "secondary mesoderm cells" are not cells at all, but 

 the products of cell degeneration, aud that in their origin and final destiny they bear 

 the closest resemblance to the "chromatin nebulae" of the lobster. 



ABNORMAL DEVELOPMENT. 

 SEGMENTATION OP THE EGG. 



In every batch of segmenting lobster eggs one is sure to meet with many irregu- 

 lar forms, and in some cases the greater number appear to be abnormal. Nuclei can 

 be detected at the surface of many of the segments, and if the egg is treated with 

 Perenyi's fluid or with an acid the dark-green segments and their nuclei contrast very 

 strongly with a milk-white coagulable substance in which they seem to be embedded. 

 Some eggs, which were laid by a lobster on August 23, after a captivity of eight weeks 

 in a small aquarium, were light-colored, but were normally fixed to the abdomen, and 

 were fertile, although the segmentation was exceptionally irregular. Sections of these 

 eggs showed an irregular distribution of cells both at the surface and throughout the 

 yolk. In some places cells appear to have been carried below the surface by over- 

 growth, and afterwards to have multiplied in the yolk. 



Eggs which are otherwise regularly segmented may contain a large superficial 

 mass of undivided yolk, as in fig. 220, plate 50. Here is a very large mass of yolk about 

 the pole of the egg — a similar one lay near it on the opposite side — and a considerable 

 number of smaller segments. When this egg is sectioned it is found that the large 

 yolk masses are nearly devoid of protoplasm, while the smaller segments contain each 

 a nucleus which shows traces of degeneration. There is no nuclear membrane, aud 

 the chromatin has assumed a very irregular form. 



It is common to find eggs with yolk uusegmented with the exception of one or more 

 small balls at the surface. Sometimes a single large segment is seen, looking as if it 

 had been pinched off, and in this and in many other cases it is evident that the egg 

 has in some way received harsh treatment. 



In one egg, rather more anomalous than usual, there was a single small spherical 

 segment at one of the poles of the elongated egg, while the remainder of the yolk was 



