169 



convinced that an interpretation may not be placed on Lepidosiren 

 similar to that which certainly can be given for the frog. Sections 

 taken through the thickened rim of the blastopore of the frog shew 

 the thickening to be due not to medullary plate, but to a mass of 

 proliferating tissue continuous with medullary plate, notochord, meso- 

 derm and endoderm alike. It is the growing point ("primitive streak") 

 and cannot be termed medullary plate any more approximately than 

 notochord, mesoderm or endoderm. Mr. Sedgwick also mentions the 

 slit-like form of the mouth of early Elasmobranch embryos as being 

 in favour of the view that the mouth is derived from the anterior 

 part of the slit-like blastopore. So far as it goes I agree that it is 

 in favour of the view, but I agree also with Mr. Sedgwick's further 

 remark "I admit that this does not constitute a very powerful ar- 

 gument" (Quart. Journ. Micr. Sc, Vol. 33, p. 571). 



In conclusion I should like to say that with Mr. Sedgwick's well 

 known views now advocated by Professor Hubrecht I am in entire 

 agreement as regards the Annelida Arthropoda, and Mollusca, but I 

 cannot help experiencing much difficulty with reference to the Chor- 

 data ; and it is this difficulty suggested afresh by Professor Hubrecht 

 in his paper (10) that I, with all deference, have ventured to present 

 in the foregoing pages in the form of "audi et alteram partem". 



List of Literature referred to. 



1) AssHETON, R., The Primitive Streak of the Rabbit; the Causes which 

 may determine its Shape, and the Part of the Embryo formed by 

 its Activity. Quart. Journ. Microsc. Science, Vol. 37, 1894. 



2) — , The Growth in Length of the Frog Embryo. Ibid. 



3) — , An experimental Examination into the Growth of the Blastoderm 

 of the Chick. Proceed, of the Royal Soc, Vol. 60, 1896. 



4) Bateson, W., Materials for the Study of Variation. London, 1894. 



5) Beakd, J., On the Phenomena of Reproduction in Animals and Plants. 

 Anat. Anz., Bd. 11, 1895. 



6) — , On Certain Problems of Vertebrate Embryology. Jena, 1896. 



7) _^ Germ Cells. Journ. of Anat. and Physiol., Vol. 37, 1903. 



8) Etcleshymer, a. C, The Location of the Basis of the Amphibian 

 Embryo. Journ. of MorphoL, 1898, Vol. 14. 



9) HuBRECHT, a. a. W., Purchung und Keimblattbildung bei Tarsius 

 Spectrum. Verhandl. d. Konink. Akad. v. Wetensch. te Amsterdam, 

 Deel 8, 1902. 



10) — , Die Gastrulation der Wirbeltiere. Anat. Anz., Bd. 26, 1905. 



11) Jablonowski, J., Ueber einige Vorgänge in der Entwickelung des 

 Salmonidenembryos nebst Bemerkungen über ihre Bedeutung für 

 die Beurteilung der Bildung des Wirbelkierkörpers. Anat. Anz., 

 Bd. 14, 1898. 



12) Kastschbnko, Anat. Anz., Bd. 3. 



