ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
455 
stomodaeal invagination appears, and the early formation of mesoderm 
at this point ; the general mode of appearance of ganglia and append- 
ages ; the body-cavity of the appendages, and the early appearance of 
mesoderm ; the formation of endodermal pouches into the appendages 
from the mid-gut, which pouches contain yolk in the embryo. The 
large “ upper lip ” of Chelifer suggests a homology with the proboscis 
of the Pycnogonida. As the first (chelate) appendages appear at the 
sides of the stomodaeum, and subsequently move forwards, and are 
innervated from part of the supra-cesophageal ganglia, they may be 
closely compared with the same parts in Arachnida. In Pycnogonids 
the lumen of the invagination of the stomodaeum is triangular in outline 
and Schimkewitsch describes a similar triangular invagination in 
Spiders. The absence of brain invaginations seems to be the only good 
objection brought by embryology against the hypothesis of the relation- 
ship of Pycnogonida to Arachnida. 
e. Crustacea. 
Development of Homarus Americanus.* — Mr. F. H. Herrick has 
found considerable resemblance between the mode of development of the 
American lobster and of Alpheus , but in the earlier stages there are 
some interesting differences. The eggs of the lobster have an average 
diameter of about 1 • 6 mm., and are invariably of a deep olive-green 
colour. The period of hatching in the summer at Woods Holl is nearly 
one hundred days. The initial stages of segmentation were not observed. 
The typical yolk pyramid structure is not present, but the entire egg 
divides into a large number of subspherical segments of irregular size. 
There appears to be a continuous migration of protoplasm from the 
central to the peripheral parts of the egg. The gastrula-phase commences 
with a small patch of cells which makes its appearance on the side of 
the egg where the cells are thickest ; a minute circular depression in it 
(which may be called the blastopore) marks the point where numerous 
cells at the surface pass into the yolk, and spread out on all sides. At 
the time of gastrulation, the great central yolk-mass is destitute of 
protoplasm, whereas in Aljpheus there is a migration of cells from the 
surface into the yolk before gastrulation begins. Later on, the position 
of the blastopore is marked by a solid, deeply-staining core of cells 
from which the cells gradually thin out on all sides. The anterior side 
of this cell-mass, which the author calls the keel, is marked by the more 
crowded condition of the cell-nuclei ; this forms the proper embryonic 
area. 
The naupliar appendages appear nearly simultaneously in tho 
embryonic area at a considerable distance in front of the keel ; they 
are at first widely separated, but after a short interval the embryo 
undergoes a marked contraction. The optic discs are represented by a 
single tier of columnar cells. The anterior portion of the keel enters 
into the abdominal plate ; the invagination of the stomodaeum occurs at a 
point between the first and second pairs of antennae. The labrum soon 
begins to grow down over the mouth, the proctodeum is established as 
an ingrowth of ectoblast on the surface of the thoracic-abdominal process 
* John Hopkins Univ. Circ., ix. (1890) pp. 67-8. 
