MUSEUM OF COMPARATIVE ZOOLOGY. 313 



Embryo a little more advanced : 15, lateral atrium ; i, mid-atrium ; e, branchial aperture ; k, 

 peripharyngeal ridge ; I, epipharyngeal ridge ; v, ganglion. 



embryo, and will describe at once the subsequent development of the 

 placenta. The inner cavity of this is at first one undivided chamber 

 into and out of which the blood of the nurse is constantly passing ; 

 but soon a singular club- or stump-shaped structure (Fig. 18, 11) ap- 

 pears in its opening, dividing this, at first incompletely, into two, and 

 projecting into the sinus system of the nurse as well as into the cavity 

 of the placenta, and serving to more effectually divert the blood into 

 the latter. Appearances seem to indicate that this organ is not derived 

 from any of the parts of the nurse or embryo, but is formed directly 

 from the blood, by the aggregation and fusion of its corpuscles. Ob- 

 servations upon this point were made so frequently and with such 

 uniform results that there seems to bo little doubt that it does origi- 

 nate in this way. No traces of a cellular structure were observed 

 in it. It is at first entirely free from all the adjacent parts, but 

 very fine threads are soon visible extending from it, and some of 

 these soon unite to the neck of the placenta and serve to anchor the 

 two together ; the terminations of the remaining threads were not 

 traced, but they seemed to float in the blood of the sinus. 



The lower end of this organ now extends downward, and joins the 

 bottom of the placenta, and soon becomes divided into a number of 

 root like portions (Figs. 19, 21, and 22,11), which separate the cavity 

 into irregular intercommunicating lacuna?, through which the blood 



