28 HERBERT SPENCER AND 



Now in the Chaetopod worms there is a larval stage 

 analogous to that of the caterpillar. We know it as the 

 trochosphere larva. It swims about in the sea by means 

 of its ring of cilia ; it has rudimentary sense organs ; it has 

 a mouth and a complete digestive tract, and it feeds 

 itself. Eventually it gives rise to the adult worm by a 

 process of development which I need not detail now. 



But you can see at once that the larva has a specific 

 form and the adult has a specific form, and that the two 

 are very different indeed. Both have to be provided for 

 in the germ-cell which is to give rise to both forms in 

 succession. 



When we examine the cell lineage of such a worm 

 as Polygordius, it is perfectly clear that the first 

 materials segregated during segmentation are those 

 destined to form the larval organs. While this part of 

 the ontogenetic process is pushed on, the materials for 

 forming the adult organs are held in reserve, and are 

 contained in only a few cells, which do not proceed to 

 further division until the larva is perfected, has fed for 

 some time, and has laid up a provision of nutriment 

 sufficient for the performance of the next phase of develop- 

 ment leading to the adult condition. 



But in a large number of marine worms a trochosphere 

 larva is produced which does not feed itself. In such 

 •cases the egg is provided with a quantity of food material 

 in the shape of yolk granules. During segmentation this 

 yolk is segregated in the large ceUs of the so-caUed vegeta- 

 tive hemisphere, from which the digestive tract is ulti- 

 mately formed. A trochosphere-like larva is arrived at, 

 but it has no functional digestive tract ; only an inner 

 mass of large yolk-laden cells, which provide the nutri- 

 ment necessary for the growth processes. In all such 

 cases adult organs begin to make their appearance early 

 in the ontogeny, and the larval stage is clearly in course 

 of suppression. In consequence, we find that the materials 

 necessary for the formation of adult organs are segregated 



