76 Jenkinson, Germinal Layers of Vertebrates. 



In the origin of the mesoderm the anomaHes are even 

 more serious. In the majority of cases it is, truly, derived 

 from 4<y, though even then, it must be remembered, this 

 cell may contain endodermal elements ; but it may and 

 frequently does happen that the mesoderm receives 

 additional elements from other sources. In Crepidnla 

 descendants of 2a, 2b, 2c, in Unto of 2a, in Aricia either 

 of 2c, 2d, or of 3(:, 3*^, in Physa of 3(5, 3^ produce larval 

 mesoderm ; in Dreissensia and Liniax^^ mesenchyme 

 arises from both second and third quartettes ; in Capi- 

 tella 3^. I and -^d. i are the pole cells of the mesoblast 

 bands, 4<3^ producing merely larval mesoderm. Long ago 

 Lankester*^ showed that in Pisidiuin the ectoderm of the 

 Gastrula could give off mesenchyme cells, and now Meisen- 

 heimer and Tonniges*^ have demonstrated that in Di'-eis- 

 sensia, Limax, and Paludina, while the mesoblast bands 

 originate from d^d in the typical manner, the material for 

 the heart, pericardium, kidneys and gonads is derived from 

 the ectoderm later on. In Aplysia, on the authority of 

 Georgevitch, though not of Carazzi,*'' 2c and 2d are the 

 primary mesoblasts, in TetJiys $d instead of 4<^, while in 

 the Turbellarians {pace Wilson) the second and third 

 quartettes alone produce mesoderm and mesoderm alone. 



Lastly, the endoderm is in Cyclas and Teredo confined 

 to quadrant D, while in Tethys it is only formed after a 

 fourth quartette of ectomeres has been given off 



Here, then, in a series of forms in which we can apply 

 the test of origin in the most decisive and unequivocal 

 manner to questions of homology, that test breaks in 

 pieces in our hands ; here, as in the Vertebrates, origin 



** Meisenheimer, J., Zeitschr. wiss. ZooL, vol. 62, p. 415 — 468, 1897. 

 *6 PhiL Trans., vol. 165, p. 1—48, 1875. 

 *® Zeitschr. wiss. ZooL, vol. 6i, p. 541 — 605, 1896. 

 * '' Georgevitch (/. c. ) describes A. depilans ; Carazzi {Anat. Ans. , vol. 17, 

 p. 77 — 102, 1900), A. limacina. 



