EMBRYOLOGY OF TEREBRATULINA. 255 



parent to allow one to watcli the eggs as they are gradually drawn to the infundibiiliform 

 orifices of the oviducts, aud to secure them as they escape by the external orifice of the 

 oviduct. In parting the shells for this purpose great care must be taken not to open them 

 too far, as the parietal wall wUl rupture, and from this opening the pei'ivisceral fluid and 

 eggs wUl escape. The eggs are not uniform either in shape or size, though a distinct gran- 

 ulated membrane, the ovishell, is formed while the egg is contained in the perivisceral cav- 

 ity. Segmentation is next seen, first as a few indentations upon the periphery of the egg, 

 until finally the whole egg presents a mass of cells, as sho-svn in Fig. 11. The egg next as- 

 sumes an oval shape, becomes cUiated, and in several eggs at this stage a dim opening ex- 

 tending to the centre midway between the opposite poles of the egg was seen — whether 

 this was the micropyle or the scar from which the egg escaped from the ovisac I could not 

 determine ; gradually the egg becomes mdened at one end, which is to be the base, or pos- 

 terior pole of the embrj-o. A long pencil of cilia sprmgs from the smaller anterior end, so 

 characteristic of the Annelid embryo. 



First Stage. From some of my dra-\vings it would seem as if the peduncular ring con- 

 sisted of the primary posterior pole of the egg, the thoracic ring widening and enclosing 

 it. At all events the thoracic ring forms the widest portion of the embr3-o, directty in 

 front of which a second fold makes its appearance. Whether this is an indication of another 

 ring I could not make out ; later, however, it merges into the base of what is to be the 

 cephalic ring, and the deep constriction that is soon to follow occurs between this fold and 

 the thoracic ring. An end view shows the thoracic ruag slightly flattened Avhile the cephalic 

 ring preserves its circidar outlme. In this condition the embryo swims slowly aljout. This 

 ends the first stage. 



Second Stage. In this stage, and one that seemed of the longest duration, a matter of 

 three or four days, the eml:)ryo is quite elongated, and is divided by a deep, transverse line 

 into two unequal halves, the head segment being the smallest. At this stage it is the 

 most active, no longer moving slowly along the bottom of the dish, or remainmg motion- 

 less, but ceaseles.sly swimming in a rapid manner back and forth through the water, and 

 abruptly turning about ; the segments expanding and contracting upon each other, and 

 the head bending from side to side, the thoracic segment often being transversely wrinkled 

 by this bendmg. A peculiarity of the embryo at this stage consists in the various external 

 outlines assumed by the embryo, presenting also different proportions, as shown in Figs. 18, 

 19, 20 and 21, drawn from the same embryo. The first definite trace of internal structure 

 now occurs in the formation of the alimentary tract, appearing as a long, narrow tract run- 

 ning from the thoracic to the cephalic ring, closed posteriorly, but blending with the cell 

 contents of the cephalic segment. A clear intersj)ace is also seen at the anterior jDortion 

 of the thoracic ring. What appeared to be an opening in the posterior end of the embryo 

 was seen in some cases. 



During all these stages, from the egg upward, great difficulty arises in making out any 

 feature of internal structure, owing not only to the minute size of the objects, but to their 

 opacity, and what little has been made out has in most cases been done hy slightly 

 compressing the embryo. 



During this active swimming stage in some embryos is seen a structure that appears to 

 be the incipient peduncular segment. In Plate viii.. Fig. 25, and Plate ix.. Fig. 78, a rounded 

 process is seen protruding from the thoracic segment, the walls of which extend as far as 



