. Text-book of embryology. Embryology. CHAP. X PODAXONIA 373 American species P. gouldii has been worked out by Gerould (1907), who also confirmed his results by work done on the European species P. vulgare. -The eggs of the American species, when they mature, are dehisced into the coelom and pass into the nephridia. They are finally laid in the sea and there fertilized. They are provided with a strong " chorion " or " yolk-membrane," which persists until the close of embryonic and the beginning of larval development. The spermatozoon penetrates this membrane through a micr
. Text-book of embryology. Embryology. CHAP. X PODAXONIA 373 American species P. gouldii has been worked out by Gerould (1907), who also confirmed his results by work done on the European species P. vulgare. -The eggs of the American species, when they mature, are dehisced into the coelom and pass into the nephridia. They are finally laid in the sea and there fertilized. They are provided with a strong " chorion " or " yolk-membrane," which persists until the close of embryonic and the beginning of larval development. The spermatozoon penetrates this membrane through a micropyle. After the eggs have been fixed in picro-sulphuric acid, it is possible to dissolve the chorion by exposing them to Laburraque's solution for two hours, and according to Gerould no harm is done to the egg itself by this treatment. The cleavage reminds us in many ways of that of Dentalium. The egg divides into the usual four macromeres, but D is, from the. Fia. 301.—Early segmentation stages of the egg of Phascolosoma gouldii. (After Gerould.) A, 4-cell stage viewed from the side. B, 8-cell stage viewed from the side. C, 8-cell stags viewed from above. A, chorion ; , polar bodies. first, very much bigger than its sisters A, B, and C. It has, in fact, five times the volume of any one of its three sisters. In the 8-cell stage a first quartette of micromeres is formed, and these are relatively large cells, as big as the smaller macromeres. In the 16-cell stage la, lb, Ic, and Id divide as usual into la^, W-, lc\ and Id^, and la^ Ib^, Ic^, and Id^ respectively. These two sets of cells are about equal in size, and they are larger than the residual macromeres 2A, 2B, and 20. Of the second quartette of micromeres which, with these macromeres, form the lower half of the egg, 2a, 2b, and 2c are small, but 2d and its sister cell, the residual macromere 2D, are both enormous and of about equal size. In attaining the 32-cell stage, the upper eight cells of the egg divide equally,
Size: 2493px × 1002px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
