10G THE COCKROACH: 



Animal Metamorphoses. 



To investigate the causes of metamorphosis, let us select from 

 the same sub-kingdom two animals as unlike as possible 

 with respect to the amount of post-embryonic change to which 

 they are subject. We can find no better examples than 

 Amphioxus and the Chick. 



The newly-hatched Amphioxus is a small, two- layered, 

 hollow sac, which moves through the sea by the play of cilia 

 which project everywhere from its outer surface. It is a 

 Grastmca, a little simpler than the Hydra, and far simpler than 

 a Jelly-fish. As yet it possesses no nervous system, heart, 

 respiratory organs, or skeleton. The most expert zoologist, 

 ignorant of its life-history, could not determine its zoological 

 position. He would most likely guess that it would turn either 

 into a polyp or a worm. 



The Chick, on the other hand, at the tenth day of incubation, 

 is already a Bird, with feathers, wings, and beak. When it 

 chips the shell it is a young fowl. It has the skull, the skele- 

 ton, the toes, and the bill characteristic of its kind, and no 

 child would hesitate to call it a young Bird. 



Amphioxus is, therefore, a Vertebrate (if for shortness we 

 may so name a creature without vertebrae, brain, or skull), 

 which develops with metamorphosis, being at first altogether 

 unlike its parent. The Chick is a Vertebrate which develops 

 directly, without metamorphosis. Let us now ask what other 

 peculiarities go with this difference in mode of development. 



Amphioxus produces many small eggs ( T ^ mm. in diameter) 

 without distinct yolk, and consequently segmenting regularly. 

 The adult is of small size (2 to 3 in. long), far beneath the 

 Chick in zoological rank, and of marine habitat. 



The Fowl lays one egg at once, which is of enormous size 

 and provided with abundant yolk, hence undergoing partial 

 segmentation. The Fowl is much bigger than Amphioxus, 

 much higher in the animal scale, and of terrestrial habitat. 



Which of the peculiarities thus associated governs the rest ? 

 Is it the number or size of the eggs ? Or the size, zoological 

 rank, or habitat of the adult ? The question cannot be answered 

 without a wider collection of examples. Let us run over the 



