THE AMERICAN LOBSTER. 
211 
In tlae 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 u in such typical forms as Alpheus and Homarus argues,” as I remarked in an 
earlier paper, u 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 Bntscliinsky (31). It seems that there can be no doubt that the 
formation of yolk cells at this early period is the last 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 stomodamm. 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- 
