172 bulletin: museum of comparative zoology. 



regard to the embryology of the gland : — (1) that it is formed from an 

 inner mesodermic and an outer ectodermic part ; (2) that the formation 

 occurs at a time when the mesoderm shows no regularity in the distri- 

 bution of its cells, these being scattered without visible ordei\ 



I have published (Waite, '98) a short abstract of some of the chief 

 points in the development more fully described hereafter. 



B. Development in the Embryo. 



In the following account of the development of the antennal glands 

 in Homarus, it should be stated at the outset that it is very difficult to 

 fix definitely the time intervals between successive stages in the em- 

 bryonic development. The determination of age in hours or even in 

 days is not of much importance. Since the rate of development is 

 known to depend largely upon temperature, the conditions of embryos 

 of the same age must vary with every station, with eggs extruded at 

 different seasons of the year, and also with eggs extruded at the same 

 date in a given locality in different years. Since, then, rate of develop- 

 ment is dependent upon many variables, the known intervals between 

 the killing of different embryos in the same series are of only relative 

 value, not being accurately comparable with the intervals of any other 

 series. Further, even in the same series, in which apparently the same 

 conditions have prevailed, there is individual variation. 



Notwithstanding these sources of uncertainty, I shall state the ap- 

 proximate age at which the several stages appear, this approximation 

 being arrived at by comparing the several series of my material with 

 one another and with the published figures of Bumpus ('91) and Her- 

 rick ('95). I shall give in each case the date of killing, which will in- 

 dicate the seasonal influences to which the development was subjected. 



The earliest embryo which it is necessary to consider in dealing with 

 the development of the antennal gland was killed on August 29, and 

 had reached the late nauplius stage (Plate 2, Fig. 15). This embryo 

 has the cephalic lobes well marked off, and the first (at. 1) and second 

 antennas (at. 2) and the mandibles (md.) well defined. The second an- 

 tenna; are biramous and reach nearly to the posterior limit of the pleonic 

 flexure. Of the post-oral appendages, the buds of the first maxillae have 

 just begun to appear. 1 



1 This stage is a little further developed than that figured by Herrick ('95) in 

 Cut 32, Plate I., which he estimates by comparison with embryos raised from seg- 



