. Journal of morphology. extend far beyond,even to the region occupied by the sucking discs. In , 18, and 19 by Morgan and Tsuda the blastopore cannot beinterpreted as extending even to the posterior border of thetransverse fold. These differences, together with the very great individualvariations, make the evidence from this source very unsatisfac-tory. For my own part, I question the entire evidence adducedfrom pathological forms. Minots (92) principal reason for believing concrescence tooccur in Amphibia is the assumption that the dorsal lip of theblastopore, or rim of the blastoderm
. Journal of morphology. extend far beyond,even to the region occupied by the sucking discs. In , 18, and 19 by Morgan and Tsuda the blastopore cannot beinterpreted as extending even to the posterior border of thetransverse fold. These differences, together with the very great individualvariations, make the evidence from this source very unsatisfac-tory. For my own part, I question the entire evidence adducedfrom pathological forms. Minots (92) principal reason for believing concrescence tooccur in Amphibia is the assumption that the dorsal lip of theblastopore, or rim of the blastoderm, becomes farther andfarther removed from the segmentation cavity. Were this thecase, it would be no more evidence of concrescence than of No. 2.] EARLY DEVELOPMENT OF AMBLVSTOMA. 389 overgrowth. A comparison of successive stages during gastru-lation shows a continual diminution of the segmentation cavity,due to the extension of the gastral cavity, the part farthestremoved from the dorsal lip being the last to Fig. 3. ^G 4 Fig. I. — Diagram showing the formation of the embryo as observed in 2. — Ditto in Coregonus and most Teleosts. Fig. 3. —Ditto in certain Teleosts (Batrachus, Amiurus, Lophius) and 4. — Ditto in Chick after Rauber (modified).The portion of the embryo () formed by differentiation in situ is represented in black ; thatlonned through backward extension, or overgrowth (*.<:.) by dots, while the extent of concrescenceforming the primitive streak is represented by the heavy line The successive positions occupied by the germ-ring, or thickened blastodermic rim, are rep-resented by t, 2, 3, 4, etc. These observations are the principal ones thus far broughtforth to support the theory of concrescence in Amphibia, andto my mind they are far from convincing. The accompanying diagrams are introduced to illustrate themodification arising in the formation of the embryo through a 390 EYCLESHYMER. [Vol. X. const
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Keywords: ., bookcentury1800, bookdecade1, booksubjectphysiology, bookyear1887
