ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 549 



yet divided into a dorsal and a ventral part, there appears a small linear 

 thickening of the epiblast. This thickening is the same on _ either side, 

 and lies horizontally and a little above the level in which the intermediate 

 cell-mass is to appear. Posteriorly this epiblastic thickening fades away, 

 but in the direction of the head it becomes more marked, and appears in 

 cross-section as a distinct semicircular clump of five to eight cells 

 adhering to the epiblast. A little further forward it becomes gradually 

 separated from the epiblast, and lies as a solid cord about midway 

 between the epiblast and the rudiment of the Wolffian body. Still 

 further forward the cord of cells acquires a lumen, and lies in contact 

 with the Wolffian body, so that it is now easily recognizable as the 

 segmental duct. The development of the circulatory system agrees 

 generally with the account given by Shipley of the same system in Petro- 

 myzon. This portion of the paper concludes with an account of the 

 development of the brain. 



In the second part the bearing of the facts of the development of 

 the Lizard on certain speculations regarding the phylogeny of the 

 Vertebrata is pointed out. 



Gastrula of Amphibians.* — Dr. Schwinck discusses the nature of 

 the gastrula in amphibian development. Bufo vulgaris, Bana temporaria, 

 and Triton alpestris were investigated. The clearest results were obtained 

 from the study of Bufo ; frog ova are more difficult. The general con- 

 clusion established is that the whole of the endoderm, including the 

 dorsal portion, arises from a differentiation of yolk-cells. The gastrula 

 of Amphibians occupies a midway position between that of Selachia and 

 that of AmpMoxus. In all, the dorsal blastopore wall is the more active, 

 and it is there that the formation of endoderm first begins, " At the 

 close of gastrulation, an archigastrula might be hypothetically formed 

 from the amphigastrula by supposing the yolk-cells to be replaced by a 

 single layer of endoderm." 



Development of Petromyzon fluviatilis.t — Prof. A. Goette has a 

 preliminary notice of his observations on the development of Petromyzon 

 fluviatilis. Gastrulation is effected as in the Amphibia ; the archenteron 

 commences with the prostoma, which lies beneath the germinal cavity ; 

 its dorsal wall becomes differentiated into ecto- and endoderm, and this 

 differentiation is continued on to the lateral parts of the thick lower 

 half. The mesoderm does not appear till gastrulation is complete, when 

 it is developed in the dorsal endoderm. This is at first multilaminate, 

 and the lower layer gives rise to mesodermal plates. Segmentation of 

 the mesoderm commences in the anterior portion of the region of the 

 trunk, and is thence continued backwards and forwards. 



The notochord is developed in the way described by Calberla ; its 

 hinder end has at first no definite termination, but is lost in the cell- 

 mass at the dorsal margin of the prostoma, where the ectoderm passes 

 into the endoderm. There is no neurenteric canal in the embryos or 

 larvae of Petromyzon ; the prostoma becomes the anus, and the primitive 

 lumen of the mid-gut is replaced by a second which arises more deeply, 

 while the primitive lumina of the fore- and hind-gut are retained. 



The spinal nerves do not arise in the way described by Sagemehl ; 

 the several rudiments of the spinal nerves become, secondarily, dorso- 



* Biol. Centralbl., viii. (1888) pp. 29-31. t Zool. Aiizeig., si. (1888) pp. 160-3. 

 1888. 2 Q 



