ON THK nBVELOPMKNT OF SAGITTA. 385 



velopmeiit, and ultimately giving rise to the ovaries and 

 testes. Soon after their appearance a pair of folds arise at 

 the anterior pole, and, growing backward, divide the archen- 

 teric cavity into three divisions, of which the middle one 

 forms the alimentary canal, and the lateral ones the coelomic 

 cavities. 



The folds push before them the genital cells for some 

 distance, and the latter ultimately come to rest in the 

 coelomic cavities, two in each side. At about this stage the 

 front ends of the coelomic spaces become sepai*ated off as 

 head-cavities, and at the same time the ectoderm of the 

 anterior end is invaginated, and meeting and fusing with the 

 endoderm produces the mouth. The blastopore comes to lie 

 a little in front of the posterior end before it closes, and by 

 the lengthening of the embryo in the shell and the general 

 compression caused thereby, all cavities become obliterated. 



The ventral ectoderm of the body and that above the 

 mouth become thickened, and many of the nuclei of the 

 former sink in and become aggregated in two ventro-lateral 

 masses against the mesoderm, forming the origin of the 

 ventral and cerebral ganglia respectively, while at the same 

 time most of the nuclei of the mesoderm form four aggrega- 

 tions, a dorsal and a ventral in each section of the mesoderm, 

 extending from the neck to the tail. The embryo assumes a 

 pronounced ventral curvature in the shell, from which it 

 escapes usually not more than two days after the eggs are 

 laid. 



At hatching the larva is not more than 1 mm. in length 

 and is rod-like in shape, tapering somewhat from head to 

 tail. There is already the rudiment of the lateral fins in the 

 tail region, and some tactile organs are present on the 

 epidermis. It is solid and contains no cavities; behind the 

 head there is a single layer of epidermis, which ventrally along 

 half the body is much thickened, and constitutes the rudiment 

 of the ventral ganglion, and behind this it is laterally thick- 

 ened to form the beginning of the fin-fold. The mesoderm 

 consists of two solid strands from neck to tail, the nuclei of 

 VOL. 46, pai;t 2. — new series, a a 



