278 PROFESSOR PAVESI. 



is to say, immediately after the nurse has developed them by 

 gemmation, and whilst they are placed on its surface like a 

 ribbon, which bears indications of a division into four pieces, 

 there appears on each side of a continuous and regular band, 

 which is placed in the middle line, and is their endostyle, a 

 very fine canal, which runs throughout their length, and, 

 arriving at the extremity, disappears under the endostyle and 

 joins its felloAv of the other side, constituting thus a true 

 vascular loop. 



These small vascular canals present in their course a vary- 

 ing diameter, and are pinched or enlarged in correspondence 

 with the constrictions and dilatations which indicate the 

 rudimentary embryos. 



The maximum diameter is a little greater than that of the 

 walls of the embryos themselves. The extremities of the 

 vascular loop continue beyond the chain of embryos to a knob 

 on the nurse formed of vitelline granules. This is the point 

 at which the heart is formed a little later. 



I have not been able to observe the actual development of 

 the heart, nor to see it until the little embryos reveal their 

 branchiae, under the form of transverse elements. Then it 

 makes its appearance a little below the buccal aperture of the 

 nurse, like a little tube, somewhat long, directed obliquely 

 towards that part w^here the embryos are placed ; by its pulsa- 

 tions it forces the vitelline globules to them. 



Since I have observed this fact in a thick section of 

 Pyrosoma, which was in consequence not very transparent; 

 and, moreover, since after a little time the pulsations of the 

 heart ceased, I was not able to follow well the course of the 

 globules in the embryos ; but they could not flow otherwise 

 than into vascular loop already described. When the em- 

 bryos present already the ganglion, the mouth, the branchiae 

 with their longitudinal and transverse elements, and begin to 

 separate themselves a little one from another and from the 

 nurse, that is to say, in the successive and later stages of 

 development of the entire system, that cord which I spoke 

 about begins to become manifest, and one can see very w'ell 

 that it is composed of two parallel and contiguous vessels. It 

 is easy, then, to understand that this cord originates in the 

 vascular loop of the rudimentary embryos, which by enlarging 

 little by little, and separating by degrees, allow the two 

 vessels to be seen in the intermediate space, thus presenting 

 the appearance of a connecting cord. 



The cord continues in the interior of the embryos, and 

 there is formed a circle, which can be recognised by the 

 rapid current of the blood-globules ; but it is easier to follow 



