278 JOHNS HOPKINS UNIVEESITY MOEPHOLOGICAL MONOGRAPHS. 



ratory nor excretory, but that it is purely a nutritive organ. I have also 

 shown that its cavity is a single chamber; that all its tortuous and 

 irregular spaces open at last into the blood spaces of the chain-salpa ; 

 that the immediate function of the placenta is to nourish its own con- 

 stituent cells from the blood which is driven into it by the heart of the 

 chain-salpa; and that the embryo is nourished by wandering placenta 

 cells which migrate into its body. 



The internal structure of the fully developed placenta is compli- 

 cated and puzzling, but it is much simpler at the younger stages which 

 are shown in Plate XVIII, in the transverse sections in Figs. 3, 4, 5 

 and 6, and in the longitudinal sections in Fig. 8. In Plate XLVI 

 three horizontal sections, Figs. 2, 3 and 4, are shown at the stage which 

 is shown, in longitudinal section, in Plate XXXV. Its boundary wall, 

 Plate XVIII, 23, is colored red in the figures, and is derived (p. 243) 

 from the lower part of the epithelial capsule. The roof, 10, is colored 

 blue and is derived (p. 269) from the somatic layer of the follicle. The 

 cavity of the placenta is filled by a spongy mass of strings of cells, which 

 are colored blue in the figures. They are derived, as Fig. 3 of Plate XI 

 and Fig. 1 of Plate XLV show, from the roof of the placenta. This mass 

 ends below in the so-called "blood-bud" or "bell-clapper," 24, which, as 

 transverse sections show, Plate XVIII, Fig. 4, 24, divides the neck of the 

 placenta into two channels which communicate with each other through 

 the irregular spaces in the upper part of the spongy mass of cells. These 

 cells are nourished by the entangled blood, and they not only grow to a 

 great size, but they also store up a yolk-like substance and thus come to 

 resemble eggs. Finally they push through the roof of the placenta into 

 the body cavity of the embryo, where they are shown in Salpa pinnata at 

 29 in Plate XVIII, and in Salpa hexagona in Plate XLV, Fig. 4, 29. 



The structure of the fully grown placenta is so complicated and 

 irregular that it is necessary to study complete series of sections, in at 

 least two planes at right angles to each other, to gain a clear picture of 

 it, but serial sections prove that it contains only one chamber, and that 

 this has no direct connection with the body cavity of the embryo, while 

 it is directly continuous with the blood system of the chain-salpa. 



I have shown, page 49, that a functional identity with the mamma- 

 lian placenta has been generally assumed, and it is not at all strange 

 that the authors who have studied it with this hypothesis in mind have 

 misinterpreted the complicated passages in its upper portion, and have 

 believed in the existence of the structure which its imaginary physiology 

 has been supposed to require. 



