326 BULLETIN OF THE 



study of this organ, Leuckart's account is again referred to, in order 

 to furnish as complete an account as possible of all that is known 

 of the evolution of all the organs of the foetus. 



According to Leuckart (page 56), "The mass of cells, which is the 

 first indication of the ganglion, and which lies upon the anterior end 

 of the branchial sac, diagonally opposite the heart, to which it is not 

 inferior in size, soon shows traces of a cavity, surrounded by very 

 thick walls, and it continues in this condition for some time. After 

 the remaining organs have gradually been developed, and histological 

 differentiation begins, the cavity of this organ becomes filled up, thus 

 transforming it into a solid mass of cells, which is easily recognized 

 by its great size, and is surrounded by a capsule. The ganglion 

 proper does not seem to exist before this stage is reached, for the 

 primitive mass of cells with the cavity which appears within it are 

 to be regarded, not as the ganglion, but as the capsule within which 

 the ganglion is to be formed." 



I have now described the way in which all the organs of the soli- 

 tary Salpa are formed, with the exception of the ovary, and the stolon 

 which gives rise to the chain ; and these can be best treated in con- 

 nection with the history of the development of the chain-salpa. 



Development of the Salpa-Chain. 



When the solitary embryo has reached the stage of development 

 shown in Figs. 24, 25, 33, s, at which time it is about one thirtieth 

 of an inch long, the tube which is to give rise to the chain appears 

 within its body, and is at first simply a cupdike protrusion of the outer 

 tunic into the cellulose test which now surrounds the embryo. This 

 cup (Fig. 25, u) is situated midway between the nucleus and the 

 placenta, upon the hamial side of the body and directly opposite 

 the heart, and its cavity is a diverticulum from the sinus system, 

 into and out of which the blood passes. Since the wall of the cup 

 is derived from the outer wall of the body cavity, while the heart 

 is upon the inner or branchial tunic, it is plain that the cavity 

 of the cup is separated from the pericardium by the width of the 

 sinus system. A small bud-like protrusion now appears upon the 

 surface of the pericardium, and lengthens so as to form a long rod, 

 which extends across the sinus and projects into the cavity of the 

 cup, as shown in Huxley's Fig. 4, Plate XVI., which is copied as Fig. 



