Fig. 165. Two views of a late embryo of Sagitta. A. from the dorsal surface. B. from the side. (After Bütschli.)
m. mouth; al. alimentary canal; v.g. ventral ganglion (thickening of epiblast); ep. epiblast; c.pv. cephalic section of body cavity; so. somatopleure; sp. splanchnopleure; ge. generative organs.
From the somatic mesoblast of the trunk is derived the single layer of longitudinal muscles of Sagitta, and part of the epithelioid lining of the body cavity. The anterior termination of the trunk division of the body cavity is marked in the adult by the mesentery dividing into two laminæ, which bend outwards to join the body wall.
The cephalic section of the body cavity seems to atrophy, and its walls to become converted into the complicated system of muscles present in the head of the adult Sagitta.
In the presence of a section of the body cavity in the head the embryo of Sagitta resembles Lumbricus, Spiders, etc.
The generative rudiment of each side divides into an anterior and a posterior part ([fig. 165], ge). The former constitutes the ovary, and is situated in front of the septum dividing the tail from the body; and the latter, in the caudal region of the trunk, forms the testis.
The nervous system originates from the epiblast. There is a ventral thickening ([fig. 165] B, v.g) in the anterior region of the trunk, and a dorsal one in the head. The two are at first continuous, and on becoming separated from the epiblast remain united by thin cords.
The ventral ganglion is far more prominent during embryonic life than in the adult. Its position and early prominence in the embryo perhaps indicate that it is the homologue of the ventral cord of Chætopoda[152].