380 W". K. BROOKS ON THE LIFE-HISTORY 



garding any significant points. Lankester's claim, that the two accounts conflict with 

 each other, seems to he the result of his desire to show that neither of them is correct, 

 but that his own very different explanation of the process is the true one; and 1, there- 

 fore, quote the words of Fol's account, for comparison with the statement which I have 

 quoted from Metschnikoff. 



lie says (22, p. 485), "Der anfangs fast kugelige Schirm breitet sich mehr nach unten 

 und aussen aus, und nimmt bald eine wirklich schirmformige Gestalt an. Dei* Rand 

 des Schirmes nimmt der Randwnlst ein, welcher sich schnell ausdehnt und zugleich 

 relative verdunnt. 



Der Magen tritt dabei verhaltnissmassig immcr mehr in die Ilohe, so dass er in den 

 Grrund einer, anfangs seichten, triehterformigen, spater tiefen, glockenformigen Hohle 

 zu liegen kommt. Letztere ist die wachsende Schirmhohle. Ein Epithel kleidet ihi'e 

 Wande aus, welches direct von der oralen Ectodermscheihe abstammt. AmMundrande 

 sieht man innner noch die Grenze zwichen Ento- und Ectoderm, welche ihrer verschie- 

 denen Beschaffenheit wegen noch unterscheidbar sind." 



For all morphological purposes it is a matter of no consequence whether the bell 

 cavity is formed by a pushing in at its centre, or by the growth of its edges, or in both 

 ways, and it is easy to understand that closely allied species may differ in this respect. 

 This difference upon a minor detail is therefore no reason for doubting the accuracy of 

 either Fofs or MetschnikofTs account. 



The youngest medusa which I obtained in the open water is shown in PI. 41, fig. 9. 

 It is peculiarly interesting on account of the simple structure of its digestive cavity, and 

 it presents a very early stage in the formation of the chymiferous tubes, the origin of 

 which has never been traced. 



It is true that Haeckel gives an account of the origin of these structures, and says 

 that they are formed by differentiation of the epithelium of the sub-umbrella; but as 

 we now know that the sub-umbrella is lined by ectoderm, no one would, at the present 

 time, believe, without very conclusive evidence, that endodermal structures originate in 

 this wa} r , although, at the time Haeckel's paper was published, such an error was not 

 unnatural. 



Haeckel says (30 b, p. 136) : :t The gastrovascular system is differentiated from 

 the cells which cover the velum and line the cavity of the bell as a sub-umbrella. This 

 differentiation takes place in such a way that, on the bell margin, at the junction of the 

 velum and the sub-umbrella, a broad strip of larger and thicker-walled cells be- 

 comes specialized as the embryonic circular tube. At the same time two similar strips, 

 crossing each other in the middle of the arch of the sub-umbrella, and joining the bases 

 of two opposite tentacles, are differentiated from the general surface of the sub-um- 

 brella.-' 



"These are the four radial canals, which, like the circular canal, are at first so wide 

 that only four small four-sided areas of the sub-umbrella remain free and covered 

 with the smaller, flatter and thin-walled epithelial cells." 



According to Ray Lankester (45) Haeckel alone has given a correct account of the 

 origin of the sub-umbrella; but I doubt whether any other embryologist would at the 

 present day credit the statement that the endodermal chymiferous tubes are formed from 



