Comparative Development. 



33 



FIG. 48. Development of the gastralaof an amphioxus (lancelet). 

 1 irst fig., parent cell. Third fig., mulberry germ. Fourth fig., mul- 

 berry germ in section. Fifth fig., invagination begun. Sixth tig., 

 bell gastrula. 



animals. It would seem that for animals to live upon 

 land, a modification of the gastrula would be neces- 

 sary, or else the parent animal would have to nurse the 

 eggs till the young animal could be hatched, that is, 

 the parent would have to be viviparous. And such is 

 the fact. All the land animals that develop through a 

 gastrula without nutritive yelk, are viviparous, while 

 all that develop through such a gastrula that are not 

 viviparous are aquatic. The hood gastrula is certainly 

 a more complex organism than the bell gastrula, and 

 it includes the highest grades of animals in its type. 

 But it also includes some very low forms, a fact sig- 

 nificant of the genetic relationship connecting the high- 

 est with the lowest animals. The following are most 

 important divisions of animals passing through the hood 

 gastrula stage: 



(a) Numerous plant animals (many sponges, medu- 

 sae, corals, siphonophores, ctenophorse). 



(6) Most worms. 



(c) Most mollusca. 



(d) Individual star animals, (viviparous species, and 

 a few others). 



(e) A few articulates (arthropoda) both crustaceans 

 and tracheates. 



00 Cyclostoma, ganoids, amphibia, mammals. 

 To the disc gastrula type belong the following: 



(c) Cuttle fish or cephalopods. 



(e) Some articulates, as millipedes, scorpions and others. 



(/) Primitive fishes (selachii), osseous fishes, reptiles, birds, (and 

 monotremes. ) 



To the last, or bladder gastrula type, belong the great body of the 

 articulates (arthropoda) crustaceans, myriapods, spiders and insects, 

 and probably a few sponges and worms. 



Of the four gastrula forms, the bell is certainly the simplest and the 

 first in point of time. The subsequent modifications from it, though 

 apparently small, involve at last important results, as will be seen 

 further on. 



The science of embryology has proved that every organized individ- 

 ual on earth, begins life as a single celled animal. All organized indi- 

 viduals of the higher and more complicated forms, pass from their sim- 



