JOURNAL OF MICROSCOPICAL SCIENCE. 



EXPLANATION OF PLATE XII, 



Illustrating Dr. Pavesi's paper on the Circulation of the 

 Blood iu Pyrosoma, especially as observed in the Embryo. 



All the figures except the last have been drawn from nature ; only those 

 organs important for giving the relations of parts are indicated. Letters 

 have throughout the same signification, that is to say — ar, vessel of tlie 

 cord which communicates with the heart ; av, vascular canal of the rudi- 

 ments of the embryos ; be, mouth of the embryos: bn, mouth of the nurse; 

 br, branchiae ; c, heart of the nurse ; c' , heart of the embryo ; c" , heart of 

 an adult Ascidian ; chr, blood-vessels of the branchiae ; cv, vascular cord ; 

 e, compound embryos ; ^ , budded embryos ; est, endostyle ; g, ganglion ; 

 n, nurse ; f, pericardium ; sp, peripheral sinus of the nurse ; ss, blood-sinus 

 of the adult embryos ; tg, gemmiferous tube or peduncle of an adult 

 Ascidian ; v, vessel of the cord which opens into the peripheral sinus of the 

 nurse ; vn, food-yolk, of the nurse. 



Fig. 



1. — Nurse with the band of four embryos, running along which is seen the 

 vascular canal ; the vitellus of the nurse is obvious near one extre- 

 mity of the embryos. X 50. 



2. — First appearance of the heart of the nurse. X 50. 



3. — Nurse with its heart iu its pericardium ; the vascular cord and its 

 continuation to the interior of the embryos are seen. X 50. 



4. — Nurse with the heart communicating with the peripheral sinus and with 

 one of the vessels of the cord ; the other vessel opens directly into 

 the sinus ; the neural surface of the first and fourth embryos can be 

 seen, and the vascular cord which is attached beneath their ganglion ; 

 between them is seen the second embryo, from the endostylic sur- 

 face, and the cord concealed behind, x 50. 



5. — The hearts of the embryos appear for the first time and coexist with 

 that of the nurse ; it is very much reduced, x 50. 



6. — One of the developed embryos when the nurse has disappeared ; tlie 

 vascular cord is atropied ; the space between the two tunics is a 

 blood-sinus, iu which the heart is seen ; the branchiae present their 

 canals traversed by blood, and communicating with the sinus; a 

 part of the branchiae is not represented, in order to expose the heart. 

 X 50. 



7. — Budded embryos, in the most developed of which is seen the vessel 

 which passes from the ganglion towards the endostyle ; a part of 

 the mother Ascidian with the gemmii'eious lube and the heart is 

 represented. X 50. 



8. — Fibro-eellular tissue of the external tunic of the vessels of the embryos. 

 X 700. 



9. — Diagram to show more clearly the course of the vessels in the series 

 of compound embryos; the arrows indicate the direction of the 

 blood in a given moment. In the figure the vessel ar represents the 

 arterj, and v the vein ; when the current is reversed, v becomes the 

 artery and ar the vein. 



