1897.] NATURAL SCIENCES OF PHILADELPHIA. 157 



The successive stages up to the 16-celled stage are shown in the 

 accompanying illustrations, Plate I. The origin of the respective 

 cells is indicated by arrows. The cells of the 4-celled stage are 

 designated A, B, C, D in the order of formation ; A', B', C, D' are 

 successively developed from these in the formation of 5-celled, 

 6-celled, 7-celled and 8-celled stages. In the same manner a, b, c, d, 

 are derived from the previous A', B', C, D', forming stages 9-celled, 

 10-celled, 11-celled and 12-celled. The next four stages are desig- 

 nated a', b', c', d', and constitute 13-celled, 1 4-celled, 15-celled and 

 16-celled stages in their respective order of formation. 



The blastula has a very small blastocoele ; the blastopore forms 

 the mouth. The cells of the endoderm take their origin from the 

 macromeres of the lower half of the blastula, while those of the up- 

 per half give rise to the ectoderm. The mesoderm can be traced 

 from the left posterior macromere. The blastula becomes bilaterally 

 symmetrical by the rise of the primitive mesoderm cells ; these lie 

 in the posterior portion on either side of the median line. Next in 

 development the endodermal cells of the blastula become invagina- 

 ted into the segmentation cavity and form the archenteron, while 

 the ectoderm grows over the invaginated portion. The primitive 

 mesoderm cells sink between endoderm and ectoderm deeper into 

 the segmentation cavity. 



In the gastrula stage the blastophore is a median ventral, longi- 

 tudinal slit ; this closes from posterior to anterior until there re- 

 mains only a small aperture. The first stage of the larva after seg- 

 mentation almost entirely fills the egg capsule, and is surrounded by 

 a zone of cilia. The body is opaque, reddish-brown in color, and 

 flattened on one side just below the cilated zone. 



In the second stage of the larva the ectoderm becoming invagina- 

 ted, forms the stomodeum or larval oesophagus ; the archenteron 

 elongates backward. There is a central opaque yolk-mass which is 

 surrounded by a layer of clear cells. This layer of clear cells is 

 thickest on the same side that the larva of the previous stage was 

 flattened. The zone of cilia persist and one pair of ocelli appear. 



The third stage of the larva has the flattened portion of the body 

 more marked than in the preceding stages. The collar originates as 

 two prominent projections on the right and left of the ventral, pos- 

 terior, median region of the body. 



The prominent features of the fourth larval stage are the increase 

 in the curve of the dorsal surface and the growth of the collar. 



