Ascidiacea 567 



small number of comparatively large eggs. Thus in the one case rearing 

 methods must start with the unsegmented egg, in the other with the 

 active tadpole larva. Only after the tadpole larvae have become at- 

 tached does treatment become the same for both kinds. 



Oviparous forms. In Ciona and in the species of Ascidia all eggs 

 found in the oviduct are ripe. In some species of Ascidia, especially 

 those that tend to become exposed at low tides to the warmer tempera- 

 ture of the air, a large percentage of the eggs in the distal part of the 

 oviduct may be dead (Berrill, 1929). Eggs from the middle or proximal 

 part of the oviduct are in any case less likely to be over-ripe. To obtain 

 the eggs and sperm the test should be removed, the wall of the oviduct 

 or sperm duct punctured with fine scissors and the germ cells withdrawn 

 in a pipette. This is much more satisfactory than maintaining the 

 parents in aquaria and waiting for them to spawn naturally. 



A similar procedure may be followed in the case of the oviparous 

 species of Molgula (and Eugyra), although the oviduct is short and 

 confined to the central part of the gonad, so that this method is likely 

 to result in the extraction of immature as well as ripe eggs. As unripe 

 eggs are more resistant generally than ripe eggs, fertilized or unfertilized, 

 this is no serious drawback, as the larvae may be segregated as soon as 

 they become active. 



Such a method used with oviparous styelids and pyurids is only occa- 

 sionally successful, and it is better to maintain the adults in aquaria 

 and collect the eggs as they are spawned and fertilized in the water. 

 In the case of Styela partita spawning occurs at sundown (Conklin, 

 1905), and this may be so for the majority. 



Viviparous forms. Viviparous species vary considerably in egg num- 

 ber and egg size. In some species almost all embryos will be at the same 

 stage of development, as in Botryllus, Polyandrocarpa, Boltenia echi- 

 nata, and Tethyum pyriforme (Berrill, 1935). In most species embryos 

 at all stages of development are to be found. In both groups embryos 

 extracted before the attainment of the tadpole stage will not continue 

 development unless certain precautions are taken. If the developmental 

 stages desired are those of the tadpole or later it is always safer to keep 

 the parents alive in aquaria until the tadpoles are liberated. Or the 

 embryos may be extracted and those tadpoles exhibiting signs of activity 

 segregated. In the first method the risk is that the parent animals will 

 not live long enough, in the second that they may not contain tadpole 

 larvae sufficiently mature. 



REARING OF EMBRYONIC STAGES 



Until the tadpole stage is passed and metamorphosis completed to 

 form a small ascidiozooid that is attached and has open siphons and 



