TRANSACTIONS OF SECTION D. 683 



sitorv monodermic stage is epiblastic in nature, and not bypoblastic, as Robinson 

 will "have it. The true hypoblast spreads out against its inner surface, much in 

 the same way as Van Beneden has described it for the rabbit. Van Beneden and 

 Julin for the bat, Heape for the mole, Selenka for the opossum, and the author for 

 the shrew. This conversion of the monodermic into a didermic stage is, however, 

 in Tupaja brought about in a stage when the dimensions of the blastocyst are 

 yet exceedinglv small. 



In conclusion the author pointed out that the early developmental stages here 

 sketched have confirmed him in the conviction that in the formation of the 

 mammalian blastocyst csenogenetic processes play a prominent part. It is by pre- 

 cocious segregation that cell-matter of epiblastic and of bypoblastic ancestry is 

 arranged into a two-layered vesicle, whereas the really formative matter out of 

 whicli the embryo will be built up is yet quiescent in the embryonic knob. 



This fact has made the mammalian blastocyst a very puzzling structure to com- 

 parative embryologists ; and we can only come to a clear conception of its real 

 nature if we are willing to acknowledge that the holoblastic segmentation of the 

 mammalian ovum is something totally diHerent from the holoblastic segmentation 

 of very many invertebrates and of Amphioxus, Being an apparently palin- 

 o-enetic feature, it is all the more misleading. And we must not wonder that the 

 fater processes of mesoblast formation have been afl'ected by the csenogenetic 

 changes just alluded to which have come about in the very early stages. 



2. On the Ancestry of the Chordata. By "W. Garstang. 



On the Structure of the Integument of Polyodon. 



By W. E. COLLINGE. 



4. On the Vertebrce o/ Amphisile. By "W. E. Collinge. 



Department of Botany. 



1. Two Irish Brown Alga;. By Professor T. Johnson. 



The author described the characters of Reinke's genus Pogotrichiim, founded 

 in 1892, and compared them with those of Litosiphon, founded in 1850 by 

 Harvey. The recently discovered plurilocular sporangia of Litosiphon Laminarice, 

 Harv., were described, and it was considered that the evidence aflbrded by their 

 position and characters with the dift'erences in the vegetative organs was sutHcient 

 to justify the continuance of Pof/otric/mm as a genus distinct from Litosiphon. 

 Lantern slides, microscopic and other illustrations, were shown. 



2. Some Chalk forming and Chalk-destroying Alga: 

 By Professor T. Johnson. 



The author exhibited and described the characters of a nuoiber of Corallinacece 

 (chalk-forming red algte) collected by him at ditJerent parts of the Irish coast 

 during the past three years. Preparations were shown of chains of spermatia form- 

 ing the antheridla in Lithophyllum (Melobesia) lichenoides, an<l attention was 

 called to their possible mode of formation. Specimens (microscopic. &c.) of shell- 

 boring algte were shown and new features in their life-history and distribution 

 described. Suggestions as to the relation of the carbonate of lime to the two groups 

 ■were made and asked for. 



