NOTES ON EMBRYOLOGY AND CLASSIFICATION. 401 



Kingdom," 'Quart. Journ. Mic. Sci.,' January, 1875, and 

 article "Animal Kingdom," 'Encyclopaedia Britannica'), 

 adopted the view, to which he still adheres (' Anatomy of 

 Invertebrates,' Churchill, 1877), that we may distinguish 

 among the higher animals the " archfeostomntous" from the 

 " deuterostomatous," the first category including those in 

 ■which the orifice of invagination persists as the mouth, 

 whilst the second category includes those in which the orifice 

 of invagination either disappears or becomes the anus, whilst 

 a secondary mouth is formed by disruption. A further con- 

 sideration of the subject and new observations led me, about 

 the same time, to the conclusion (see " On the Invaginate 

 Planula of Paludina," ' Quart. Journ. Micr. Sci.,' April, 

 1875) that we have no ground for assuming that such a 

 substitution of a secondary for a primitive mouth has taken 

 place, since it is very possible (and, indeed, probable) that 

 the orifice of invagination of invaginate Planulae is in its 

 origin not a mouth at all, but simply the necessary accom- 

 paniment of the invagination, destined normally to close up, 

 as do other orifices of invagination (optic and auditory 

 vesicles, vertebrate nerve-tube). Accordingly, I proposed to 

 speak of the orifice of invagination, by means of which inva- 

 ginate Planulse acquire their endoderm, as simply the " blas- 

 topore," leaving thus the question of its relations to mouth 

 and anus open for further inquiry. The view as to the his- 

 torical relations of delaminate and invaginate Planulae which 

 I was thus led to adopt amounted to tiiis : that, starting 

 from the condition of a hollow polyplast, a vesicle bounded 

 by a single layer of cells, the second condition, viz. that of a 

 vesicle with a wall formed by two layers of cells, could be 

 attained in two ways — 1. More rarely by delamination. 2. 

 More usually by invagination, the blastopore or orifice of 

 invagination closing up, and thus rendering the two Planulse 

 identical in every respect. From this point of reunion the 

 two Planulae proceed on a common path, mouth and anus, or 

 in Zoophytes mouth only, being formed by new growth and 

 disruption. 



This preliminary sketch is sufficient to enable me to make 

 clear the distinction between what I might, for the sake of a 

 name, call my "Planula theory," and Haeckel's "Gastrula" or 

 " Gastrsea theory." Haeckel's speculations were first sketched 

 out in his 'Monograph of the Calcareous Sponges,' and were 

 published shortly before the article in the ' Annals and 

 Mag. Nat. Hist.' above mentioned, though the substance of 

 this article had been previously given in my lectures, and 

 was in no way influenced by the closely similar doctrine 



