W. K. BROOKS ON THE GENUS SALPA. 241 



SECTION 5. The Significance of the Symmetry of the Salpa Embryo. 



The result of this comparison is, to my mind at least, the establish- 

 ment of a strong presumption that the fertilized egg of salpa is, like that 

 of clavelina, a potential embryo, and nothing short of the most complete 

 and satisfactory evidence to the contrary should prevent us from 

 believing that it is destined, in course of nature, to become an actual 

 embryo, after the analogy of other eggs. 



Seeliger's identification of the first three planes of segmentation with 

 the regions of the body of the potential clavelina is based upon careful 

 study, and his description inspires confidence in the accuracy and value 

 of his conclusions ; and even if the precise relations between the cells of 

 his eight-celled stage and the body of the embryo should prove to be 

 somewhat different, there can be no doubt whatever that these cells and 

 the cleavage planes which produce them out of the egg do stand in some 

 definite relation to the potential tunicate. 



The bilateral symmetry of the segmenting egg of salpa and its 

 similarity to that of clavelina lead us to the same belief, that the eight- 

 celled stage in salpa is a potential salpa, and that its eight cells and the 

 planes which separate them are definite and fixed ; nor is there any 

 evidence that this regularity in the arrangement of the blastomeres is 

 ever lost. My inability to control the planes of my sections gives a 

 semblance of irregularity to my figures of the older stages, but there is 

 nothing in my own sections or those figures by Salensky to show that 

 the arrangement is actually irregular at any stage. 



The segmenting egg of salpa is an embryo, beyond question ; either an 

 embryo which normally becomes a modern salpa, or one which became 

 an adult in the remote ancestors of the modern animals. If the latter is 

 the case, and the body of the modern salpa arises in great part from 

 another source, and if the blastomeres of the segmenting egg are chiefly 

 nutritive in function rather than formative, it is difficult to understand 

 why their regular arrangement should have been retained, for cells 

 whose function is nutritive rather than vital are generally characterized 

 by irregularity. 



The early stages of salpa are such as to cause us to approach, in an 

 extremely critical and exacting frame of mind, the view that, in the 

 older stages, " Die Blastomeren mehr und mehr in den Hintergrund 

 treten und wohl kaum irgend eine Rolle bei der Bildung der Organe 

 spielen " (5, p. 390). 



