64 PROFESSOR E. RAY LANKESTER. 



A second invagination forms the true mouth and the stomo- 

 daeum^ (so-called pharynx or Vorderdarm). The hypoblastic 

 archenteron or gastrula's stomach formed by the first invagi- 

 nation gives rise only to the middle portion of the alimentary 

 canal (Mitteldarm or mesenteron) and the liver. 



Here, too, as in other cases pointed out by Haeckel, the 

 first development .of the mesoblast is in the region of the 

 blastopore-margin, but here too no certain information can 

 be given as to the origin of all its elements, in regard to the 

 question of their derivation from ectoderm or endoderm. 



The formation of the Gastnila of the Calcareous Sponges. — 

 Metschnikoff", Oscar Schmidt, and F. E. Schvilze have re- 

 cently written on the development of the Calcareous Sponges, 

 and have shown that Haeckel's statements on this matter 

 were not entirely correct. In the first place the formation 

 of the Gastrula of sponges by delamination, and the subse- 

 quent breaking through of the mouth, has to be given up. 

 As Haeckel observes, this is so much the better for the 

 Gastrsea-theory, for the delaminate mode of development, 

 though, as I have pointed out, reducible to the same iiltimate 

 significance as the invaginate mode of development, is yet a 

 cenogenesis. Haeckel doubts whether delaminate Gastrulse 

 really exist at all, and thinks that early invagination, will 

 be found to occur even in the Hydroids on careful study. 

 This point should be at once investigated. Haeckel admits 

 in reviewing the work of recent observers his fundamental 

 error as to the mode of origin of the Gastrula of Calcareous 

 Sponges, but rightly enough maintains that in the most 

 essential features they have proved his statements to be jus- 

 tified. The observations of Metschnikofi" are shown to be 

 erroneous, as well as his conclusions by the later work of 

 Schmidt and Schulze ; in fact, he takes endoderm for ecto- 

 derm. Oscar Schmidt's observations are correct in most 

 points, but i7icomplete, and his inferences quite erroneous, as 

 appears from Schulze's work. Schulze confirms Haeckel 

 in the most important point, namely, that a true gastrula 

 with its two primary germ-layers does make its appearance 

 in the development of the sponges. 



The four works on the development of Calcareous Sponges 

 furnish an exceedingly interesting series illustrative of the 

 genesis of error in such studies. Haeckel seized upon the 

 really great and fundamental fact that the Sponges are no 

 Protozoa, but that, like those of Coelenterata, their eggs give 

 rise to a sac — the Gastrula — with a wall of two cell-layers, en- 



1 This term and its correlative ' proctodaeum ' I propose for the oral and 

 anal invaginations. 



