ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 355 



been observed by Leydig in various animal cells, and by Scbafer in 

 tbe young ovarian ova of the fowl, seems to be a different phenomenon ; 

 for there is not then any limiting layer, nor any folds of membrane, 

 but rather a condensation of the cellular reticulum. 



The author is unable at present to say certainly vphat bearing his 

 observations have on those lately made by Balbiani, Fol, and others, 

 which have only come to his knowledge since his paper was completed. 



Origin of Metameric Segmentation.* — In this suggestive essay 

 A. Sedgwick discusses the origin of the metameric segmentation and 

 some other morphological problems. To the hypotheses which he has 

 already suggested — that the mouth and anus in most of the higher 

 groups have been derived from some such elongated opening as is now 

 seen in the Actinozoa, that the somites of segmented animals are 

 derived from a series of pouches of the archenteron comparable to the 

 pouches now found in Actinozoid polyps and Medusae, and that the 

 nephridia are derived from sjDecialized parts of these pouches — he now 

 adds a fourth, that the tracheae are not developed from cutaneous 

 glands of a worm-like animal, but are rather to be traced back to 

 simple ectodermal pits, represented to-day by the subgenital pits of the 

 Scypho-medusae, by the pits into the cephalic ganglia of Arthropod 

 embryos, and by the canal of the central nervous system of the 

 Vertebrata. 



It is pointed out that the essence of all these propositions lies 

 in the fact that the segmented animals are traced back not to a triplo- 

 blastic unsegmented ancestor, but to a two-layered Coelenterate-like 

 animal with a pouched gut, the pouching having arisen as a result of 

 the necessity for an increase in the extent of the vegetative surfaces 

 in a rapidly enlarging animal. In framing these hypotheses, support 

 is found in facts of exactly the same nature as those which have 

 been used in tracing the evolution of the nervous and muscidar 

 tissues. 



The difficult problem of the homology of the mouth and anus with 

 the mouth of the Coelenterata is first attacked ; their mode of develop- 

 ment is shown to be coincident, and their relations to the archenteron and 

 the neural surface of the body to be similar. The fact that in various 

 higher forms the blastopore gives rise in some to the mouth and in 

 others to the anus is to be explained by the supposition that the blasto- 

 pore primitively gave rise to both mouth and anus, and that its 

 specialization as a larval organ has caused the variability of its sub- 

 sequent history. This doctrine is supported by the characters of the 

 mouth of the existing Actinozoa where the two ends are open and the 

 middle closed, and by the fact that in the primitive Peripatus the 

 elongated blastopore does give rise to both mouth and anus. If this 

 be true, it follows that Balfour was right in thinking that the stomo- 

 daeum and proctodseum are not in all cases completely homologous. 



The common ancestor of the Triploblastica and Actinozoa may be 

 supposed to have been a diploblastic bilateral form with an enlarged 

 oral surface, and an elongated mouth which, by the adhesion of its 



K * Quart. Jouru. Micr. Sci., xxiv. (1884) pp. 43-82 (2 pis.). 



2 B 2 



