280 PROFESSOR PAVESI. 



and here also circulates, and then returns to the first and 

 afterwards to the nurse ; m the same manner it circulates in 

 the third and in the fourth embrj'os, thus constitviting the 

 general current. Thus, the same phenomena repeat them- 

 selves for a certain time, but the heart stops, and then 

 reverses the direction of its pulsations ; then, in place of com- 

 mencing its movements from the portion which I shall call 

 " auricular " — i. e. the open extremity — it commences from 

 the portion which I shall call " ventricular ;" and thus the 

 vessel that communicates with the heart is changed from an 

 artery into a vein, and the current is reversed in all the 

 vessels of the embryos. By taking great pains I have suc- 

 ceeded in w^atching the circulation of the embryos during 

 several hours siiccessively, changing frequently the drop of 

 Avater in which they were placed. 



INIr. Huxley refers the nutrition to the contents of the 

 ovisac, which, he says expressly, " must serve as a great 

 reservoir of material for the developing embryo after birth ;" 

 and he adds, immediately afterwards, " the contents of the 

 ovisac, therefore, though neither a food-yolk nor a placenta, 

 serve the purpose of both." From what I have stated, it is 

 clear that the nutrition of the embryos takes place, on the 

 contrary, through the nurse; and it is so true that it is at its 

 expense, that it becomes reduced little by little, and finally 

 disappears entirely. This nurse, or rather vitellus of nutri- 

 tion, may be compared to that of birds, fishes, &c., and the 

 two vessels which form the cord for the embryos to the 

 omphalo-mesenteric vessels or to the hepatic vein, which 

 finally absorbs all the vitellvis. Always bearing in mind that 

 in Pyrosoma this vitellus of nutrition is common to all the 

 four embryos, and is placed aw^ay from them in a true nurse. 



Later, but while the heart of the nurse still exists, having 

 become much smaller, each embryo also commences to pre- 

 sent a heart, which beats independently, and apparently 

 before it is traversed by a current of blood. It is seen late- 

 rally at the level of the mouth, but at the opposite surface, 

 and it opens freely into the space which exists between the 

 two tunics of the embryo. Now, then, all the five hearts 

 beat, but those of the embryos are not synchronous with that 

 of the nurse, and they commence their pulsations when 

 that of the nurse pauses, but they are synchronous with one 

 another, and always present the phenomena of alternation. 

 However, these alternating pulsations of the hearts of the 

 embryos are variable — sometimes slow, sometimes very quick, 

 and with a different duration of pause. On one occasion 

 I counted 74 pulsations, then a pause, a change in the direc- 



