W. K. BROOKS ON THE GENUS SA.LPA. 277 



worthy that the destruction of the protoplasm which takes place in all 

 the other species, is not to be observed in Salpa democratica. In place 

 of it, we find scattered in the protoplasm of the blastomeres small, round 

 bodies which color deeply in carmine. I can give nothing more exact 

 regarding their origin and significance, and will only add that these 

 bodies are also found in the follicle cells around the blastomeres." 



While he is thus in doubt regarding Salpa democratica, the passages 

 which I have quoted show that in all other species he regards these 

 bodies as products of the degeneration of the blastomeres ; but his view 

 is not very clearly stated, for on page 103 he says that they have a 

 great resemblance to the deutoplasm of other eggs, with which they are 

 perfectly analogous as regards their later history. I am not able to dis- 

 cover what it is which he compares with other eggs, for of course neither 

 the blastomeres nor the follicle cells are eggs. 



SECTION 7. The Fate of the Portion of the Visceral Layer which invests 



the Blastomeres. 



The origin of this layer of follicle cells is shown at 8 in Plate X, 

 Figs. 3 and 5. Its cells elongate and multiply until it forms a thick wall 

 around the visceral mass, as shown at 8 in Plate XLII, Figs. 5, 6, 7 and 

 8, and also in Plates XIII, XIV and XXII. After the blastodermic 

 lining of the pharynx and perithoracic structures is formed, this layer 

 lies between it and the body cavity, as shown in Plates XVI and XVII. 

 It persists as a distinct tissue until the embryo is well advanced, but 

 finally its cells become separated, as shown in Plate XVIII, and wander 

 through the body cavity as migrating follicle cells. The disintegration 

 begins at the bottom of the embryo and gradually extends upwards, and 

 Plate XVIII, Fig. 7, 8, shows some of these in the act of separation. 

 After they become free they become vacuolated, as shown in Plate XX, 

 Fig. 5, and the blood corpuscles, or mesenchyma cells, settle upon their 

 surfaces and penetrate their substance, and form over them a network of 

 fibers, Plate XXI, Fig. 2, which persists as a loose, honeycombed tissue, 

 Plate XIX, Fig. 9, after the cells have entirely disappeared. 



SECTION 8. The Placenta. 



I have shown (pp. 48-52) that the function of the placenta of salpa 

 is quite unlike that of the mammalian placenta ; that it is neither respi- 



