G. A. DREW ON YOLDIA LIMATULA, SAY. 21 



slit in a muscular partition that projects dorsally into the cavity of 

 the auricle. The arrangement of this partition is such as to tend to 

 stop the backward flow of blood into the gills. The ventricle is per- 

 forated by the intestine. Where each auricle joins the ventricle, there 

 is a marked constriction and, internally, a muscular septum perforated 

 by a single opening. When the ventricle contracts, this septum 

 probably contracts and more or less completely closes the opening in 

 it. 



Two blood-vessels leave the ventricle, one anteriorly on the left 

 side of the intestine, the other posteriorly, beneath the intestine. 



EMBRYOLOGY. 



The eggs of Yoldia limatula, are about .15 mm. in diameter, of a 

 chocolate brown color, and very opaque. They are not encumbered by 

 any kind of envelope. The polar bodies are lost soon after they are 

 formed. The first cleavage, which takes place about two hours after the 

 eggs are fertilized, results in the formation of sub-equal blastomeres, 

 Fig. 24. 



In the next cleavage, Fig. 25, the smaller blastomere divides equally 

 and the larger unequally. The eight-celled stage is formed by a cleav- 

 age at right angles to the two preceding planes of cleavage, and the 

 two sets of four cells each come to lie in the position indicated by 

 Fig. 26, the last division being in the plane of the paper. 



One of these cells is considerably larger than the other seven and, 

 with the beginning of the next cleavage, begins to be inclosed by the 

 other cells, Fig. 27. The small cell formed by the unequal division of this 

 large cell, Fig. 28, becomes a surface cell. What becomes of similar 

 cells in later divisions has not been determined, but, from sections, 

 it appears probable that the next small cell, cut from the large cell, 

 Fig. 29, crowds in over the tip of the large cell and divides into two, 

 Fig. 37. 



About this time the outer cells, or at least part of them, acquire 

 cilia, Fig. 30, and the embryo begins to roll around on the bottom of 

 the dish. It finally rises to the surface of the water and swims freely. 



Just what internal changes take place during this interval is 

 rather hard to determine. Surface cells in the region of the blasto- 

 pore became especially active, dividing and crowding in around the 

 large internal cell, which now divides into nearly equal cells, Fig. 39. 



