3i6 THE UNFERTILISED EGG AS A [pt. iii 



forms equally with adults. The majority of marine invertebrates leave 

 the egg as minute ciliated larvae, which are quite incapable of holding 

 their own in currents of any strength. Hence it is only forms which 

 have got rid of the free-swimming ciliated larval stage, and which 

 leave the egg as organisms of considerable size and strength, that can 

 establish themselves as fresh-water animals. This is effected most 

 readily by the acquisition of yolk — hence the large size of the eggs of 

 fresh-water animals — and is often supplemented by special devices." 



Here is an explanation for the well-known paucity of eggs in fresh- 

 water plankton. In certain cases it is possible to induce an embryo 

 to skip the larval stage which it should normally pass through. Thus 

 Child could abolish the free-swimming larval stage in the ascidian 

 Corella willmeriana, simply by removing the eggs from the parental 

 atrial chamber {p¥L j'^.) to normal sea-water (/>H 8-4). 



Giard had also noticed the discrepancy in egg-size between closely 

 related marine and fresh-water forms, and had classed it among those 

 cases where like adults have unlike larvae ("Poecilogony"). The 

 classical instance is perhaps that of the shrimp Palaemonetes varians, 

 one variety of which {microgenitor) lives in the sea near Wimereux 

 and has eggs 0-5 mm. diam. (32 1 per female) and another of which 

 {macro genitor) lives in fresh water at Naples and has eggs 1-5 mm. 

 diam. (25 per female). Giard has reviewed this subject in a very 

 interesting paper. "Dans un groupe determine", he said {(Euvres 

 diverses, p. 18), "la condensation embryogenique va en croissant des 

 types marins aux types d'eau douce ou terrestres." 



The correlated proposition, namely, that the fresh-water forms 

 generally lay fewer eggs than the marine ones, is illustrated by the 

 following instances collected by Carpenter: 



No. of eggs laid per female per annum 



A 



Another reason for the poverty of fresh-water fauna was suggested 

 by von Martens who pointed out that the fresh-water climate, with 

 its periods of desiccation and freezing, was much more severe than 



