DEVELOPMENT. 



19 



to the accounts given below under each family or order. Here 

 we will only remark that there is considerable variability in the 

 mode of formation of the organs. The pharynx, atrium, diges- 

 tive organs and pericardium are usually derived from the epi- 

 cardial process of the pharynx, while the ganglion in the 

 synascidians usually develops from the endoderm, but it may 

 arise from the mesoderm (Pyrosoma) or from the ectoderm 

 (Thaliacea). In the Botryllidae the endoderm of the parent 

 does not participate in the budding process. 



Development. In most monascidians (except Cynthia, etc.) 

 the eggs are fertilised in the sea or in the atrium and undergo 

 their whole development outside the body of the parent. This 

 is also the case in Doliolum and in the Appendiculariae. In the 

 synascidians on the other hand the early development usually 

 takes place in the atrial cavity or in incubatory pouches of it. 

 In the salps the egg undergoes its early development in the 

 ovary ; in the later stages it emerges into the atrial cavity but 

 remains connected with the parent by the placenta. 



The eggs are frequently laid in their follicle, which is somewhat com- 

 plicated. It is formed of two layers (Fig. 14), the outer of which consists 

 of vacuolated cells ; these are prolonged into papillae, and help to float 

 the egg in the sea. The inner layer consists of follicle-cells which have 

 migrated inwards, and are called the test-cells because formerly they were 

 supposed to give rise to the test of the adult. The two layers are separ- 

 ated by a structureless chorion. 



The development generally 

 leads to the formation of a 

 free-swimming tailed larva, 

 the tadpole larva, by means of 

 which the species is distributed 

 over a wider area. The tad- 

 pole larva is nearly always 

 formed in the rrionascidians 

 (it is absent in some species of 

 Molguld) and in the synas- 

 cidians (absent in Pyrosomd). 

 It is also found in Doliolum, 

 but not in the salps. 



The eggs usually have but 



little yolk. In the synascidians they are however richer in yolk, 

 and in Pyrosoma the cleavage is actually maroblastic 



FIG. 14. Mature egg from the oviduct of dona 

 intestinalis (after Kupffer). c follicle cells 

 (foam-like cells) ; d chorion ; e test-cells ; 

 / ovum ; x gelatinous substance. 



