210 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



The relative position of the mouth and antennae is illustrated by cuts 28 and 29. 

 I will add the account given in my paper on the development of Alpheus, where the 

 lobster was also included, since it applies to the higher Crustacea generally : 



Before the first antenme are folded, when they are distinguished as dense patches of cells, some 

 eggs show the primitive mouth as a minute circular pit, lying uearly on a line drawn between the 

 centers of these proliferating cell areas, hut, so far as my observation goes, never distinctly in front of 

 them. The relative positions of the mouth and first pair of antenna shift very rapidly during the early 

 period of their growth, before the fully developed egg-nauplius stage. The pit elongates and becomes 

 a transverse furrow, and by the time the first pair of antennae are clearly marked off as rounded buds, 

 and before the second pair are raised into folds, the mouth is on a line with the first of these append- 

 ages. When the second antennse are elevated into folds the mouth is behind the buds of the first j>air, 

 or on a line drawn between their posterior margins (94, p. 442). 



At a stage before the appearance of eye pigment, represented in fig. 232, plate 51, a 

 diffuse but conspicuous patch of ectodermic cells is developed, similar to what I have 

 already described in Alpheus (94, p. 114). In case the egg is oblong this patch lies 

 at one pole about 90° behind the embryo. 



A stage just previous to the appearance of eye-pigmeut is seen in fig. 234. The 

 forked telson now covers the labrum and is reaching up in front of the brain. 



The relation between the age and size of the embryo under normal conditions — 

 the eggs having been laid in summer — may be seen by comparing cuts 23-38 with 

 table 18. Most of these are from the same batch of eggs. 



Eye pigment is developed in about four weeks, and the rate of growth of the 

 embryo can thenceforward be gauged by the increase of the pigmented area (cuts 

 35-38). 



At a stage before the concentration of the embryo has begun, a little earlier than 

 that shown in cut 29, Bumpus (30) has described and figured what has the appearance 

 of a rudimentary pair of preoral appendages. These are elongated folds lying parallel 

 with the convex border of the optic lobe, and separated from it by a slight furrow only. 

 They are very transitory, disappearing completely after a brief interval. They can 

 be seen in some of my preparations, but are not shown in the drawings. 



HISTORY OF THE YOLK CELLS. 



In my paper on the development of Alpheus I have devoted a chapter to the 

 "Origin and history of wandering cells in Alpheus," and I have little doubt that what 

 is said there of Alpheus is generally true of the lobster and of most decapoda. 



In Alpheus, as in the lobster, a certain number of cells, 30 to 40, are budded off 

 from the blastosphere, and form what I have called the primary yolk cells. Wander- 

 ing cells, or those which enter the yolk and move about in it, have a triple origin, 

 namely, from the blastoderm, the invaginated cells, aud the ventral plate. It should also 

 be added that both the process of multiplication by indirect cell division and that of 

 dissolution or degeneration of protoplasm take place simultaneously in the wandering 

 cells. 



During the egg-nauplius period there is a rapid diminution of the wandering cells, due to cell 

 disintegration and emigration to those parts of the embryo where mesoblastic organs are being laid 

 down. The history of the wandering cells in Alpheus is largely the history of the early develop- 

 ment of the mesoblast and entoblast. The endoderm makes its appearance as a distinct cell layer 

 during the egg-nauplius period, and takes the form of a narrow sheet of rather large cells, between 

 the yolk and the rudimentary heart, near the body wall. In the space corresponding to the heart, 

 blood corpuscles can already be detected, besides scattered mesoblast cells. Both the latter and the 

 entoblast are derived from the wandering cells which come out of the yolk (94, p. 408). 



