THE EVIDENCE FROM DEVELOPMENT. 



'77 



not merely as being at once the lowest fish and Vertebrate, but as 

 evincing in its development a most marked likeness to that of the 

 sea-squirt, whose manner of entrance upon the stage of life we have 

 just studied. The lancelet is a little fish .attaining a length of one 

 or two inches, and found inhabiting sandy coasts in various parts of 

 the world. Its body is pointed at either extremity, and,, save for a 

 narrow fin bordering the upper and part of the lower surface of the 

 body, no traces of the appendages commonly seen in fishes are 

 to be found. This fish occupies the position of a very singular and 

 anomalous member of the Vertebrate series. Unlike most of its 

 congeners, it has no skeleton or backbone, a mere soft: and gela- 

 tinous chord, termed the notochord, existing in the place of and 

 representing the spine. It has no paired fins or limbs ;. it wants a 

 heart; it has no skull or brain; and its organs of sense are represented 

 by mere pigment-spots for 

 eyes ; whilst the mouth pos- 

 sesses a series of filaments (c) 

 probably subserving the sense 

 of touch. This little animal 

 would seem thus to hover, 

 as it were, on the outermost 

 confines of Vertebrate exist- 

 ence. Its adult characters 

 resemble the rudimentary 

 traits of other Vertebrates ; 

 and in respect of its entire 

 structure, and still more so 

 of its development, it may 

 be said to be a connecting 

 link between Invertebrates in 

 general and sea-squirts in par- 

 ticular on the one hand, and 

 the Vertebrate sub-kingdom> 

 on the other. 



Like all other animals 

 above the very lowest, the 

 lancelet's history begins with 

 the production of the germ or egg-(Fig. 92, i), which exhibits in its es- 

 sential structure the closest similarity to that of the sponge or ascidian. 



The first changes to be witnessed in the developing egg of the 

 lancelet consist in the complete division (Fig. 92, 2, 3) of its substance. 

 Segmentation of the egg of the lancelet is on an exact parallel with 

 that of the egg of the sponge or the sea-squirt. We shall presently 

 note that this segmentation is also imitated, completely or in part, 

 in higher forms of life. As in the sponge, the " blastoderm " is duly 



N 



FIG. 92. DEVELOPMENT OF LANCELET. 



