382 THE FORMS OF TISSUES . [ch. 



diagram. It would be a curious and interesting investigation to 

 ascertain, in a large number of frogs' eggs, all at this stage of 

 development, the percentage of cases in which these various 

 arrangements occur, with a view of correlating their frequency 

 with the theoretical conditions (so far as they are known, or can 

 be ascertained) of relative stability. One thing stands out as 

 very certain indeed : that the elementary diagram of the frog's 

 egg commonly given in text-books of embryology, — in which the 

 cells are depicted as uniformly symmetrical quadrangular bodies, — 

 is entirely inaccurate and grossly misleading*. 



We now begin to realise the remarkable fact, which paay even 

 appear a startling one to the biologist, that all possible groupings 

 or arrangements whatsoever of eight cells (where all take part in 

 the surface of the group, none being submerged or wholly enveloped 

 by the rest) are referable to some one or other of thirteen types or 

 forms. And that all the thousands and thousands of drawings 

 which diligent observers have made of such eight-celled structures, 

 animal or vegetable, anatomical, histological or embryological, are 

 one and all representations of some one or another of these thirteen 

 types : — or rather indeed of somewhat less than the whole thirteen, 

 for there is reason to believe that, out of the total number of 

 possible groupings, a certain small number are essentially unstable, 

 and have at best, in the concrete, but a transitory and evanescent 

 existence. 



Before we leave this subject, on which a vast deal more might 

 be said, there are one or two points which we must not omit to 

 consider. Let us note, in the first place, that the appearance 

 which our plane diagrams suggest of inequality of the several 

 cells is apt to be deceptive ; for the differences of magnitude 

 apparent in one plane may well be, and probably generally are, 

 balanced by equal and opposite differences in another. Secondly, 

 let us remark that the rule which we are considering refers only 



* Cf. Rauber,NeueGrundlagez.K. der Zelle, Morph. Jahrb. vin, 1883, pp. 273. 

 274 : 



"Ich betone noch, dass unter meinen Figuren diejenige gar nicht enthalten ist, 

 welche zum Typus der Batrachierfurchung gehorig am meisten bekannt ist....Es 

 haben so ausgezeichnete Beobachter sie als vorhanden beschrieben, dass es mir 

 nicht einf alien kann, sie iiberhaupt nicht anzuerkennen." 



