130 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



Uljanin (7), p. 124, characterizes the method of comparison which 

 Grobben employs as irrational, inasmuch as he bases it upon purely 

 imaginary changes in the anatomy of the fully developed animals, and 

 makes no use of embryology. 



He says that Grobben overlooks the fact that the atrium or cloaca of 

 Doliolum does not correspond to that of Pyrosoma and the other Tuni- 

 cates, inasmuch as there are, in the Doliolum embryo, (p. 67) no peri- 

 thoracic tubes like those of the Ascidians, since the cloaca of Doliolum is 

 formed as a single median unpaired invagination of the ectoderm. 



It is not impossible that there may be a pair of lateral perithoracic 

 tubes in Doliolum before the median cloaca and its aperture are formed, 

 for his observations were made upon entire embryos, and no sections of 

 the structures in question are figured. In Anchinia, which Uljanin 

 justly regards as the nearest relation of Doliolum, Barrois has shown (10), 

 pp. 226, 230, 236 and 242, that the atrial structures of the sexual animals 

 arise in the buds as paired lateral ectodermal invaginations, and that 

 their history is exactly the same as the primitive history in the Ascid- 

 ians, so that Uljanin must either deny the homology between the atrium 

 of Doliolum and that of Anchinia, or else he must recognize its homology 

 with that of the Ascidians and Pyrosoma. Even if his observations are 

 accepted as final, his deduction by no means follows. 



In some echinoderm Iarva3 the coelomic pouches separate from the 

 gut before they separate from each other, while in most cases they are 

 distinct from each other before they become separated from the gut, yet 

 all embryologists regard them as homologous, and it is vastly more 

 probable that the ectodermal rudiments of the perithoracic tubes of 

 Doliolum meet on the middle line before the invagination takes place 

 than that the atrium of Doliolum is a new structure. 



While history gives ample reason for his statement, p. 124, that the 

 facts of comparative anatomy may be distorted or misrepresented, all 

 naturalists know that anatomy often proves homology and furnishes 

 a key to embryology. Thus mammalian teeth and the flat bones of the 

 mammalian cranium are held to be dermal scales, although mammalian 

 ontogeny gives no record of their phylogeny. So too the mammalian 

 body cavity is held to be a series of coelomic pouches from the gut, and 

 the mouth of a starfish is held to be strictly homologous with the mouth 

 of a sea-urchin, although in the one case it is the same as the larval 

 mouth, while in the other it breaks through on the left side of the larva. 



In all these cases we reconstruct the primitive ontogeny from the 



