THE AMERICAN LOBSTER. 211 



Iu the lobster there appears to be this difference, in that the primary yolk cells 

 are for the most part, if not wholly, disintegrated before invagination occurs, and 

 take no part in development. This can not be shown to be the case in Alpheus. 



I have spoken of the formation of the primary yolk cells by tangential division, in 

 Alpheus and other forms, as a process of delamination, on the ground that they repre- 

 sented a primitive endoderm, and that the egg with primary yolk cells corresponds to 

 the planula stage of coelenterates. I first called attention to this mode of origin of 

 yolk cells in decapod Crustacea in my paper on Alpheus (94, p. 400), and found that 

 in the lobster they arose by transverse division from the blastospheric cells or from the 

 peripheral cell layer (since there is no true blastosphere in this egg). The budding of 

 these cells, moreover, begins before the outwardly migrating cells have reached the 

 surface and completely surrounded the yolk. The regularity with which this process 

 occurs " in such typical forms as Alpheus and Homarus argues," as I remarked in an 

 earlier paper, " for its presence in allied species where it has possibly been overlooked." 

 A precisely similar origin and speedy dissolution of yolk cells has been recently 

 described in Gebia by Butschinsky (31). It seems that there can be no doubt that the 

 formation of yolk cells at this early period is thelast trace of a process which was once 

 of importance, but the role which they play now must be an exceedingly minor one. I 

 have never found more than 28 of these cells in the large egg of the lobster (cut 22). 

 Here, while it may be admitted that they are phagocytes or yolk digesters, the impres- 

 sion which they make upon this large mass of material is insignificant, and they are 

 themselves soon disintegrated and become a part of the general food stock. It is pos- 

 sible that they are the remains of a primitive hypoblast, that they once played a more 

 important part as digesters of the yolk than they do at present, and that this function 

 was usurped by the mesendoderm formed at the time of invagination. The term trans- 

 verse fission instead of delamination should, however, be used in speaking of this process 

 (94, see pp. 400 and 419), since no true delamination occurs and nothing certain is 

 known about the origin and meaning of this process in the decapod Crustacea. 



DEGENERATION OF CELLS. 



I have discussed the subject of cell degeneration in my paper on Alpheus (94, pp. 

 425-431) and need not refer to the facts again in detail (see figs. 237, 240, 241, plate 52). 

 The degeneration of cells in the ovary has already been mentioned (p. 152). In the 

 embryo this breaking down and absorption of cells into the common yolk mass is first 

 seen in the primary yolk cells, and afterwards in the mesendoderm, where it soon 

 becomes one of the most striking, and at the same time most puzzling, of all the 

 varied phenomena presented by the developing embryo. If we examine a longitudinal 

 section of the egg nauplius of the lobster, we find not a few chromatin balls, but a 

 meteoric swarm of granulated bodies and naked chromatin grains coextensive with 

 the embryo and reaching a considerable distance into the yolk amid the scattered 

 mesodermic cells, but perhaps most abundant, as in Alpheus, in the neighborhood of 

 the stoinodreum. A long, nebulous train of yolk spherules and granules extends 

 forward a considerable distance in front of the mouth and is especially marked in the 

 region of the optic disks. The labrum and the folds of the appendages which contain 

 solid yolk cores abound also in these peculiar granulated bodies. They occur in less 

 numbers in connection with the mesendoderm cells, which have at this stage traveled 

 through the greater part of the egg and form a series of irregular sacs filled with yolk. 

 These yolk masses, with their surrounding sheet or advancing column of cells, corre- 



