RECENT ADVANCES IN SCIENCE 403 



of the Cord, and on the Behaviour of Frog Embryos " (ibid.) ; 

 Kude, " On the Development of the Nerve End-organs in the 

 Ear of Trigonocephalus japonicus " (ibid.); Johnson, "The 

 Branchial Derivatives of the Pied-billed Grebe, with Special 

 Consideration of the Origin of the Postbranchial Body " (Jour. 

 Morph. vol. xxxi. June 191 8) ; Riddle, " Further Observations 

 on the Relative Size and Form of the Right and Left Testes 

 of Pigeons in Health and Disease, and as influenced by 

 Hybridity " (Anat. Rec. vol. xiv. May 191 8). 



It is pointed out by Hill, " Some Observations on the Early 

 Development of Didelphys aurita (Contributions to the Em- 

 bryology of the Marsupialia, V.) " (Quart. Jour. Micro. Sci. 

 vol. lxiii. April 191 8), that in the only two Marsupials whose 

 embryology has been fairly fully investigated, namely Dasyurus 

 and Didelphys, development takes place according to a common 

 plan. Moreover, as far as is known, the development in Mac- 

 ropus and Perameles is also essentially similar. These facts 

 justify the conclusion that a common mode of development 

 characterises the Didelphia as a group. The outstanding feature 

 of this appears to be the production of a unilaminar blastocyst 

 whose wall is divisible into two polar areas, one formative and 

 the other non-formative. The former gives rise to the embryonal 

 ectoderm and the entire entoderm of the blastocyst, while the 

 latter provides the extra-embryonal ectoderm. Two new terms 

 descriptive of the condition of the blastocyst are proposed : 

 Phanerotypy, to designate the condition in the Ornithodelphia 

 and Didelphia in which " the formative cells are freely exposed 

 and constituted from the first part of the blastocyst wall, just 

 as those of the Sauropsida form part of the general blasto- 

 derm " ; and Cryptotypy, to designate the condition in the 

 Monodelphia where " the formative cells are completely hidden 

 or enclosed by the tropho-ectodermal mantle." The unseg- 

 mented ovum in Didelphys is potentially polar in constitution 

 since the first cleavage results in the formation of the parent 

 cells of the formative and non-formative regions respectively. 

 In " The Formation, Rupture, and Closure of Ovarian Follicles 

 in Ferrets and Ferret-Polecat Hybrids, and Some Associated 

 Phenomena " (Trans. Roy. Soc. Edin. 191 8) Robinson has re- 

 corded observations made on a large number of animals whose 

 ovaries were in all stages of activity and also in the ancestrous 

 condition. In the ferret we have an animal that only ovulates 



