REPRODUCTION — TROCHOSPHERE. 
381 
Fig. 716. — Median section of Gastriila 
stage of Vivipara, seen from left side, 
greatly enlarged (after Erlanger). 
bl. blastopore; bl.g. blastoporic groove ; 
a. archenteron or primitive digestive 
cavity ; b. blastoccel or segmentation 
cavity ; ec. ectoderm ; en. endoderm ; w. 
mesoderm ; c, coelom ; v. velum. 
Archenteron, or primitive digestive cavity, and its opening at the 
nutritive pole is the Blastopore (/3Aao-To?, a bud ; Tropos, a passage) or 
elongated primitive mouth of the 
Gastrula; it usually becomes gradually 
closed, the middle part forming the 
foot, an invagination in its anterior 
part constituting the stomodmum or 
oesophagus, and the proctodmum or 
anus arising in its posterior part, both 
communicating with the Archenteron 
or digestive cavity. 
The ectodermic outer envelope of the gastrula, and its invaginated 
endoderm, enclose between them a cavity, the blastoccel, which 
eventually becomes the blood space or Hmmocoel (af/xa, blood ; koiAos, 
cavity), and acqirires chiefly from the endoderm a cellular tissue, 
the Mesoderm (/xeo-o?, middle; Sep/m, skin), which becomes split up 
into numerous sinuses and separates into a somatic or exterior and a 
splanchnic or internal layer, originating the reproductive and ex- 
cretory organs, the muscular system, the heart, and circulatory 
system generally. 
Contractile sinuses arise in the 
cephalic region and in the walls of 
the body, a large posterior sinus or 
caudal vesicle may also be developed, 
which contracts alternately with the 
somatic or cephalic vesicles, and tem- 
porarily serves to circulate the fluid 
through the body before the developing 
heart becomes functional. 
The Trochosphere (rpoyds, a hoop ; (r<^aipa, ball) stage still retains 
a bilateral symmetiy of the organs, the 
torsion of the body and the approxima- 
tion of the two ends of the alimentary 
canal not having yet been accom- 
plished. It is remarkable for the 
barrel shape of the embrjm and the 
possession of a cilial investment which 
is most distinctly developed as a 
preoral girdle or ridge, which forms a characteristic feature of 
Fig. 717. — Embryo of Agrioliiuax 
agrcstis (L.), greatly enlarged (after Van 
Beneden and Windischmann) showing the 
well developed pulsatory caudal vesicle. 
/./. labial lobes ; c.v caudal vesicle ; 
in. mantle ; oni. ommatophores ; f. foot ; 
a.t. anterior tentacles. 
The arrow indicates the direction of 
the rotation of the embryo within the egg. 
Fig. 718. — Trochosphere of Vivipara 
(enlarged after Biitschli). 
in. mouth ; 7 a velum ; st. stomach ; 
a. anus ; f. foot ; in. mantle and shell. 
