DEVELOPMENT OF THE OVUM. 351 
may be left behind in the central yolk to form the yolk nuclei, or, what 
is probably the more primitive condition, these are formed by subse- 
quent immigration from the blastoderm. 
The next process is the appearance of differentiation among the similar 
cells of the blastoderm. Over a special area—the ventral plate—(cf. 
Astacus) the cells increase in number and become cylindrical in shape ; 
over the rest of the egg the cells flatten out and become much thinner. 
In the middle of the ventral plate a slight groove is formed by rapid 
multiplication of the cylindrical cells. This represents the disguised 
gastrulation, the open roof of the groove being the much-elongated 
blastopore. The surrounding cylindrical cells unite over this open roof, 
the groove usually flattens out, and thus we have formed a two-layered 
germinal streak which spreads forwards and backwards over the egg, 
and early exhibits externally transverse division into segments. The 
upper layer is the ectoderm ; the lower includes the rudiments of both 
mesoderm and endoderm. 
Meanwhile another very important event has taken place. We saw 
that while the cells of the ventral plate increased in depth, the remain- 
ing cells flattened out laterally; at the point where the two kinds of 
cells unite, on either side of the ventral plate, a double fold arises. The 
two folds unite over the surface of the ventral plate, forming a mem- 
branous arch over it. The internal fold is called ‘‘ amniotic,” the 
outer ‘‘serous,” from their resemblance to the similar envelopes in the 
embryos of higher vertebrates. The folds take no direct part in the 
development of the embryo. 
We must now return to the germinal streak. The gastrula groove 
may persist as a tube after closure of the blastopore, but it is usually 
compressed by the ectoderm, or never exists as a distinct cavity. The 
greater part of the lower stratum of the germinal streak consists of 
mesoderm. This becomes divided into successive segments at each 
side, each containing a primitive coelomic cavity, perhaps continuous 
with the gastrula cavity. The endoderm arises as paired clusters of 
cells, found only at the anterior and posterior ends of the primitive 
streak. These clusters increase rapidly and form long endodermal 
streaks, which curve downwards so as to enclose the yolk. The streaks 
meet and fuse, first ventrally and later dorsally, thus constituting the 
mid-gut. The yolk nuclei previously mentioned have meanwhile 
increased rapidly, forming yolk cells which absorb the yolk. These 
cells are included in the endodermic mid-gut, and there break up. As 
the endoderm grows round the yolk, it is accompanied by a layer 
(splanchnic) of the mesoblast. Fore- and hind- gut are formed by 
invaginations which fuse with the mid-gut. 
In the later stages of development the primitive coelomic pouches 
lose their cross partitions, become filled with mesenchyme cells, and 
practically obliterated. The body cavity of the adult is formed by the 
appearance of lacunze amid the cells of the mesenchyme. 
The trachez arise as segmentally repeated invaginations of the ecto- 
derm. The openings of the invaginations form the stigmata. From 
the hind-gut arise the Malpighian tubules, which are therefore ecto- 
dermic. The development of the other organs is similar to that of 
the Crustacea. 
