Primitive Cell-layers of the Embryo. 337 



osteology and the embryology of higher Vertebrata have long 

 made it clear that the vertebrate mouth belongs to the series 

 of visceral clefts ; but the significance of this in the comparison 

 of Vertebrata and Invertebrata has yet to be fully appreciated. 

 The identification of the neural and ha3mal aspects of Verte- 

 brata and Vermes, in the light given by this demonstration of 

 Kowalewsky's as to the distinct character of the mouth in the 

 two cases, must lead to most valuable results *. 



The triple basis of histological differentiation, the nerve- 

 centres, the alimentary tract, the blood-lymph-spaces, the 

 segment-organs, the prostomial and metastomial regions being 

 recognizable as homogenous under varied adaptative modifi- 

 cations throughout the Triploblastica, is it not probable that 

 other parts may still further exhibit that unity of type of the 

 included groups which forms our hypothesis? Whilst it 

 is necessary always to be on guard against mistaking homo- 

 plastic agreements such as clearly must and do existf for 

 homogenetic agreements, yet, since the working hypothesis 

 must be that of uniformity, as the simpler, we ought to assume 

 homogeny or unity of type as explaining similarity in organs 

 until research proves it necessary to regard this or that particular 

 case as due to coincidence of adaptative causes. Hence it may 

 fairly be suggested that the appendages of Triploblastica, 

 appearing under two chief forms as locomotive and respiratory, 

 (external gills) are homogenous throughout the series. 



Such an hypothesis opens a very large field for discussion; 

 but within certain limits it will not, perhaps, meet with strenuous 

 opposition. The gills of Mollusca generally, of Brachiopods, the 

 tentacles of Polyzoa, and the gill-tufts of Annelids — again, the 

 locomotive appendages of Annelids and Arthropods — or, again, 

 the external gills of Vertebrata (embryo Selachians, Batra- 

 chians, &c.) and those of Annelids, offer themselves as likely 

 enough to prove homogenous ; but since many further embryo- 

 logical inquiries have to be made, and no doubt will be made 

 in consequence of these possibilities presenting themselves to 

 the imagination of many students of embryology, it will not now 

 be useful to discuss them upon the limited evidence at hand. 



Note. — Professor Hack el, in the final part of his newly 

 published splendid monograph of the calcareous Sponges, 

 has entered into speculations on the significance of the poly- 

 plast and planular stages of development and the development 



* 1 am indebted to my friend Anton Dohrn for first drawintj my atten- 

 tion to some of the legitimate consequences of Kowalewsky's observations 

 as to the mouth of Amphioxiis. 



t Ann. Nat. Hist. 1870, vol. vi. ("On the use of the term Homology"). 

 Ann. (h Mag. N. Hist. Ser. 4. Vol.yix. 22 



