NOTES ON EMBRYOLOGY AND CLASSIFICATION. 399 



(Dr. Wiegert) regards Unna's " Pockeiikorper" only as 

 serving to separate the vesicles from the rete Malpighii, and 

 thus to properly enclose them, " zur Abkapselung dient.'* 

 In addition, Dr. Wiegert thinks that Dr. Unna's " Pocken- 

 korper^' is found only at the periphery of the pustule. I 

 need hardly remind the reader that, according to our de- 

 scription above, I fully agree with Dr. Weigert's interpreta- 

 tion ; and I would only mention that Auspitz and Basch 

 (quoted by Dr. Neuman in his ' Textbook of Skin Diseases,' 

 translated by Dr. Pullar, London, 1871, p. 74) knew the 

 condition of human smallpox when '^ the pustular contents 

 are enclosed, as if by a capsule, by two layers of unnucleated 

 epidermic cells.'' 



Notes on the Embryology and Classification of the 

 Animal Kingdom : comprising a Revision of Specula- 

 tions relative to the Origin and Significance of the 

 Germ-layers. By E. Ray Lankester, M.A., F.R.S., 

 Professor of Zoology and Comparative Anatomy in 

 University College, London. With Plate XXV. 



I. — The Planula Theory. 



Preliminary. — The object of the present essay is to give, 

 in a concise form, the actual phase which those specula- 

 tions have assumed, which I first put forward in an article 

 entitled, "On the Germinal Layers of the Embryo as the 

 Basis of the Genealogical Classification of Animals," pub- 

 lished in the ' Annals and Mag. of Nat. Hist.,' May, 

 1873. The points of chief importance in that article were 

 the indication of three grades of developmental complexity 

 in the animal kingdom — the horaoblastic, limited to the 

 Protozoa; the diploblastic, reaching no higher than the 

 Zoophytes or Coelentera; and the tripoblastic, embracing 

 all the higher animals which differ from the Zoophytes 

 built up by the modification of two primary cell-layers, in 

 the fact that a third cell-layer appears between these two, 

 and gives rise to muscles, body-cavity, and blood-vascular 

 systems. The precise origin of this third germ-layer, as well 

 as its exact relation to body-cavity and haemolyraph vessels, 

 was pointed to as a matter requiring further observation and 

 consideration. Further, in this article it was shown that both 



