306 BULLETIN OF THE 



the channel through the heart, so that there will no longer he an un- 

 obstructed passage. 



The muscles contract successively in order, from one end of the 

 heart to the other, and the middle layer is thus thrown into a wave, 

 which sweeps along the roof of the lower layer and drives the blood 

 before it, and before the first wave has travelled from one end of the 

 heart to the other a second follows it, and so on. After this has gone 

 on for some time the muscles begin to contract in the reverse order, 

 and the waves start from the other end and the direction of the cur- 

 rent is reversed. The cause of this change or the meaning of the 

 reversal is as yet by no means clear, although after the pulsations 

 have continued for some time in one direction numbers of blood- 

 globules may be seen crowded together in certain organs, such as the 

 elseoblast ; and these block up the sinuses until they are set free by 

 the change in the direction of the current. 



In our species the blood-channels are in all cases sinuses, which arc 

 parts of the body cavity and have no special walls, although several 

 writers insist that in certain parts of the bodies of other species the 

 blood Circulates in true vessels lined with epithelium. 



Embryology of the Solitary Salpa, — Female. 



We are now prepared to enter upon the history of the development 

 of Salpa. Tli is is rendered somewhat complicated by the fact that 

 the two forms, besides differing considerably in structure, are devel- 

 oped in totally different ways; and before the embryo of the second 

 generation has completed its development within the body of its 

 parent, the formation of the third generation begins within it: accord- 

 ingly at certain stages we are compelled to study an embryo going 

 through one series of changes, and within this another embryo differing 

 in form, and undergoing an entirely distinct form of development. 



On account of the manner in which the two forms of development 

 overlap, it is somewhat difficult to select a point at which to begin 

 our account. The fertilization of the egg seems to be the best point 

 of departure, although some parts of the description cannot be clearly 

 understood until the account of the development of the male has been 

 read. 



The Egg: its Fertilization and Segmentation. — At the time when the 



