LIFE-HISTORY 485 



In this book different kinds of animals are used to illustrate 

 different biological ideas, and as the Ascidians have not, for 

 instance, much in the way of habits, we use them to illustrate, 

 inter alia, the turns and twists of life-history. Everyone is 

 familiar with the contrast between caterpillar and butterfly, or 

 between tadpole and frog, but a much more striking contrast is 

 that between the larval Ascidian — free-swimming and energe- 

 tic — and the sedentary, sluggish, somewhat nondescript adult. 

 Let us briefly discuss the extraordinary life-history. 



The egg of an Ascidian is usually a microscopic transpar- 

 ent sphere with little or no yolk. It is interesting to find that 

 in some cases there are relatively large eggs with a consider- 

 able quantity of yolk, which develop within the body — usually 

 until the tailed larvae are hatched. Here, as in many other 

 cases, there are " oviparous " and " viviparous " types — a dis- 

 tinction which really means that the eggs of the former are 

 hatched outside of the body and those of the latter inside. 

 But the great majority of Tunicates are oviparous. 



In the majority there is probably cross-fertilisation ; in 

 Ascidia there is often self-fertilisation ; in Ciona, though ova 

 and spermatozoa are ripe at once, self-fertilisation is rare. 



The fertilised ovum divides and redivides, and a hollow ball 

 of cells (a blastnla) is formed. The ball of cells becomes in- 

 dimpled, so that a two-layered sac of cells (a gastrula) results. 

 This becomes elongated, like a barrel, with the cavity (the 

 primitive gut or archenteron) and an opening (the blastopore) 

 posteriorly. Around the posterior opening, and extending 

 forwards along the dorsal surface, the central nervous system ap- 

 pears as a neural plate, which becomes a neural groove, 

 which becomes a neural canal. In the same median plane, 

 but at a lower level, a rod of cells is separated off, along the 

 dorsal wall of the primitive gut, which becomes the supporting 

 axis or "notochord " of the posterior region. The development 

 therefore in essential features is similar to that of Amphioxus 

 or a vertebrate, but we must not go further into detail — an 

 explanation of the development of a Tunicate is to be found 

 in most of the text-books. Suffice it to say here, that two or 

 three days after fertilisation the Ascidian larva is hatched — like 

 a Vertebrate reduced to the bare essentials. It lives for a short 

 time, swimming freely in the open sea, like a minute transparent 



