332 ELEMENTS OF COMPARATIVE ZOOLOGY. 



segmentation is visible externally; in the vertebrates it is 

 not so plainly shown, but it nevertheless exists. The trunk 

 muscles (see p. 11) are thus arranged; the spinal nerves 

 and the vertebrae correspond to the muscle segments, as do 

 also certain blood vessels (intercostals), while in their early 

 history the kidneys are segmen tally arranged. 



On the other hand, the lower worms show no traces of 

 segmentation, while the molluscs show it to a very slight 

 extent.* In the echinoderms there is a repetition of 

 ambulacra and ambulacral plates, but this is supposed to be 

 different in its origin from that in the segmented animals. 



All animals above the Protozoa reproduce by eggs. 

 These eggs, when carefully studied, are found to agree in 

 their essential characteristics. Each, in fact, is a cell 

 (p. 320) containing a nucleus; but to these essentials other 

 structures shell, white, yolk, etc. may be added. Each 

 egg, under proper conditions, is capable of growing into a 

 form like that which produced it. The essential condition 

 is that a peculiarly modified cell, the spermatozoan, unites 

 with the egg, and then the compound cell is capable of 

 development, f Keduced to its simplest terms, the process 

 of development may be briefly stated thus : 



After union with the sperm cell (fertilization) the egg 

 divides again and again, the result being the formation of 

 a large number of cells, all connected together, which later 

 arrange themselves in layers (p. 318), and then develop into 

 organs. This type of reproduction is known as sexual 

 reproduction, since egg-cell and sperm-cell are produced 

 by animals of different sexes. 



* The gills, kidneys, and heart of the Chitons (p. 273) and the 

 Nautilus (p. 267) are supposed to present indications of segmentation. 



f In a few cases, as in the honey-bee, the eggs can develop without 

 union with a spermatozoan. 



