40 C. M. CHILD 



The amount of nutritive material in the egg and its physical 

 condition, water content, etcetera and very probably its chemical 

 constitution must also play a part in determining the size re- 

 lations between egg and larva. In .Arenicola the posterior 

 region grows to a considerable size before reduction of the 

 head occurs, while in Nereis posterior growth apparentlj^ occurs, 

 largely at the expense of the head region after the earlier stages. 

 Apparently the supph' of nutrition in Arenicola suffices for the 

 demands of all parts up to a later stage than in Nereis. Alore- 

 over, the Arenicola larva, even in the three-segment stage of 

 figure 49, is still almost completely opaque because of the small 

 'yolk' granules distributed through the protoplasm of all cells, 

 and is able to continue its development to a stage of five or six 

 segments without external nutrition, but, during this develop- 

 ment, the granules gradually disappear and the protoplasm of 

 the young worm is highly translucent. In Nereis, however, the 

 nutritive supply of the egg is used up much earlier, practically 

 no visible traces of it remaining in the fully developed three- 

 segmented larva, and, in the absence of food from without, 

 development ceases. Summing up, the facts of observation and 

 experiment indicate that the general size relations between egg 

 and larva, and the larval proportions are matters of amount, 

 availability, and perhaps efficiency of food supply in the egg for 

 the metabolism of each species and of the relations of metabolic 

 rate between different parts. 



The course of development in other segmented animals 



\Miile the data are still too fragmentary to permit the formula- 

 tion of a general theory of segmental development, various facts 

 are at hand which suggest that the course of events in other 

 forms is more or less like that in the annelids. In segmented 

 forms, generally, the order of segment-formation is the same as 

 in the annelid, and in the teleosts and amphibia, the only verte- 

 brate groups studied in this way as yet, a secondary posterior 

 growing region of high susceptibihty appears, at least in the 

 development of the tail. Somites may arise anterior to this 

 growing region, doubtless through physiological isolation, and 



