644 INVEETEBEATA chap. 



Or, the old pharynx, oesophagus, and rectum degenerate, leaving the 

 new pharynx and rectum in connection with the old intestinal coil. 

 In the first case the compound organism separates into two persons 

 by the sphtting of the common pieces of oesophagus and intestine, 

 and this splitting takes place in such a way that the old intestine 

 and rectum remain connected with the new pharynx, whilst the old 

 pharynx enters into connection with the new intestinal coil. In the 

 second case the person is " rejuvenated " by the substitution of a new 

 pharynx and rectum for the old structures. 



It will be thus seen that the budding of Urochorda calls up the 

 same kind of problem as does the budding of Polyzoa, but in an even 

 more acute form. In the Polyzoon bud two totally distinct organs, 

 the circle of lophophoral tentacles, which are protrusions of the 

 ectoderm, and the gut which in the larva is formed from endoderm, 

 arise from the same ectodermal rudiment in the bud and are known 

 collectively as the " polypide." This polypide degenerates from time 

 to time, and is replaced by a new one developed in the same way as 

 the old. In the larva of Urochorda the atrial cavity and the nervous 

 system arise from the outer layer, but in the bud they originate from 

 the inner layer. In stolonial budding the inner layer of the bud 

 arises from the parental endoderm, but in pallial budding it arises 

 from the parental ectoderm. Finally, as we have just seen, in 

 Diplosomidae three distinct bud rudiments coalesce to form a new 

 individual, and the part of the individual produced by one of, these 

 buds can degenerate and be replaced hke the " polypide " of I'olyzoa. 



How are these facts to be reconciled with the doctrine of germ 

 layers? If the distinction between ectoderm and endoderm be the 

 first and fundamental differentiation of egg substances, how can an 

 ectodermal organ, like the nervous system in Urochorda, be produced 

 from endoderm ; or, vice versa, how can an endodermal organ, like the 

 gut in Polyzoa, be produced from ectoderm. 



Hjort's suggested explanation of this anomaly is extremely plausible. 

 He first of all gets over the difficulty that the inner layer of the bud 

 is formed from endoderm in Glavelina, Bistapha, Perophora, but from 

 ectoderm in Botryllus. The double-walled vesicle, he maintains, is to 

 be regarded as the starting-point of the bud as a new organism, and 

 is equivalent to the ovum, which is the same whether it appears in 

 ectoderm or endoderm. But the outer wall of the bud is in all cases 

 formed from the outer ectoderm of the parent, which is a highly 

 specialized tissue committed irrevocably to producing cellulose, and 

 which is therefore incapable of being modified into nervous tissue. 

 The ectoderm of the embryo, on the contrary, consists of compara- 

 tively undifferentiated cells, capable of plastic modification in many 

 directions. The inner layer of the bud, whether derived from 

 epicardium or atrial membrane, consists of cells in a plastic condition, 

 and therefore gives rise to all the organs of the " blastozooid " except 

 the cellulose-producing ectoderm. In the bud of the Polyzoon, if we 

 follow an analogous course of reasoning, the ectodermal outer layer 



