waite: antennal glands in homarus americanus. 173 



In an embryo of this age I find in sections that the basal region of 

 the second antenna is completely filled with cells having certain distinct- 

 ive characters (Plate 2, Fig. 18, ms'drm.), which serve to differentiate 

 them from the outer wall (ecdrm.) of the appendage. These axial 

 cells are oval or slightly elongate, with rounded outlines, and less uni- 

 form in size and less densely granular than the rectangular ectoderm 

 cells (ecdnn.), which form the outer wall of the appendage. The nuclei 

 of the axial cells are more oval than the nearly spherical nuclei of the 

 ectoderm cells. In some sections it is to be seen that this plug of axial 

 cells is connected with similar plugs of cells in the base of the first an- 

 tenna and of the mandible by means of a single layer of cells immedi- 

 ately beneath the ectoderm of this region. 



According to recent works on the formation of the germ layers and of 

 the appendages in Decapods, — I refer especially to Reicheubach ('86, p. 

 24, Taf. IX. Fig. 84, and Taf. X. Fig. 121), Bumpus ('91, p. 245, Plate 

 XVIII. Fig. 6), and Herrick ('95, p. 209), — the cells which form the 

 axis of the appendage are clearly mesoderm, in the generally accepted 

 meaning of the word. 



In embryos one day older (Plate 2, Fig. 16), in which the first max- 

 illae (mx. 1) are marked off, a small number of these axial mesodermic 

 cells in the base of the second antennae have become differentiated from 

 the rest. They are destined to become the deeper part of the antennal 

 gland. These cells are nearly on a level with the general surface of the 

 body. They are larger than the remaining mesodermal cells, and have 

 the cytoplasm less densely granular, especially toward the centre of the 

 cell, and their nuclei also are larger and stain much more deeply with 

 haematoxylin. The nuclei are likely to be eccentric, and in many cases 

 they are surrounded by a clear zone of non-granular or very sparsely 

 granular cytoplasm (Plate 2, Figs. 19, 20, 21, 22, sac. trm.). 



I believe that there is at first only one of these differentiated cells in 

 each antenna, although I have not found this condition in any of my 

 sections. My reason for thinking so is that in several cases in which 

 only two nuclei are present I have found these close together, and with 

 their adjacent sides somewhat flattened (Plate 2, Fig 19, sac. trm.), as 

 if recently divided. I have not found, however, any mitotic phenomena 

 which might serve as proof of a recent division. On the other hand, it 



mentation stages (see his Tables 17 and 18, p. 56) as 14 to 16 days old. With this 

 as a basis, the embryo of my Figure 15 would probably be between 15 and 17 days 

 old. It falls between the Stages N and O of Bumpus ('91, p. 248, Plate XIV.), 

 and is a little further developed than Reichenbach's ('86, p. 47, Taf. III. Fig. 10) 

 Stage G of Astacus. 



