1 78 CHAPTERS ON EVOLUTION. 



formed ; the infolding of this blastoderm and the formation of the 

 pocket-like "gastrula" (Fig. 92, 4, and Fig. 89, 4) taking place 

 exactly as in the development of the sea-squirt. Furnished with 

 its eyelash-like cilia, this gastrula-lancelet swims freely in the sur- 

 rounding water. Not a trace of its vertebrate character can be 

 observed at this stage. It might be the forward progeny of a worm, 

 or might be ranked as a developing snail ; whilst if it were alleged to 

 be an embryo star-fish, or a baby sea-squirt, the zoologist would pro- 

 bably own his inability to say which assertion was correct, or most in 

 accordance with the appearance of this curious organism. 



The succeeding course of events to the gastrula stage brings an 

 elongation of the body, and from the inside or pocket-like cavity of 

 the gastrula (4, d) the digestive tube of the future lancelet (5, d) is 

 seen to be gradually fermed. Then, also, appear the first marks and 

 traces of its vertebrate relationship, and of its kinship with the aris- 

 tocracy of the animal kingdom. The flattened aspect of the body now 

 shows a tendency to develop two ridges or projections, which soon 

 meet and unite in the middle line to form a tube (Fig. 92, 5, b], enclosing 

 the nervous axis. This nervous tube remains open for a time in the 

 lancelet, as depicted at o, Fig. -92. The body of the young fish 

 now assumes somewhat of the appearance of a flattened cylinder. 

 It resembles closely the young sea squirt, and, like the latter, possesses 

 in its back region, a rod-like body, the notochord (5, n). In the 

 lancelet, however, the notochord extends completely from head to 

 tail. The identity of the two developing bodies may be best 

 demonstrated by a comparison of their longitudinal sections in Figs. 

 89 and 92 (5, 5), where the arrangement of parts and organs is seen to 

 be essentially similar. 



The next change results to the fore-part of the body, where the 

 throat is seen to become cleft or perforated by the gill-slits a sea- 

 squirt feature again being apparent in this latter phase of develop- 

 ment It is equally curious to note that similar clefts to be more 

 specially alluded to hereafter appear in the development of all other 

 Vertebrates, including man ; these clefts in fishes bearing gills, but in 

 reptiles, birds, and mammals becoming obliterated. Ultimately the 

 free-swimming lancelet assumes habits of more staid character. The 

 notochord, which in most other and higher Vertebrates is replaced by 

 the spine, remains in the lancelet as the permanent representative of 

 the backbone, just as in Appendicularia (Fig. 90), among the sea- 

 squirts, the " urochord " persists throughout life. And with the 

 appearance of the systems and organs characteristic of its adult 

 existence, the preparatory stages of lancelet life may be regarded as 

 having been completed. Thus it is certain that the development of 

 the lancelet, whilst clearly that of a vertebrate animal, is also seen 

 to produce a low type of vertebrate organisation, and to present 



