Royal Society. 495 



and are succeeded by a smaller and more numerous set ; several sets 

 thus successively come into view, the vesicles of each succeeding set 

 being smaller than the last, until a mulberry-like structure has been 

 produced, which occupies the centre of the ovum. Each of the ve- 

 sicles of which the surface of the mulberry-like structure is com- 

 posed contains a pellucid nucleus ; and each nucleus jjresents a nu- 

 cleolus. 



In the uterus a layer of vesicles of the same kind as those of the 

 last and smallest set here mentioned makes its appearance on the whole 

 of the inner surface of the membrane which now invests the yelk. The 

 mulberry-like structure then passes from the centre of the yelk to a 

 certain part of that layer, (the vesicles of the latter coalescing with 

 those of the former where the two sets are in contact to form a 

 membrane,) and the interior of the mulberry-like structure is now 

 seen to be occupied by a large vesicle containing a fluid and gra- 

 nules. In the centre of this vesicle is a spherical body having a 

 granulous appearance, and containing a cavity apparently filled with 

 a colourless and pellucid fluid. This hollow spherical body seems 

 to be the true germ. The vesicle containing it disappears, and in 

 its place is seen an elliptical depression filled with a pellucid fluid. 

 In the centre of this depression is the germ, still presenting the ap- 

 pearance of a hollow sphere. The germ separates into a central and 

 a perii^heral portion, the central portion occupies the situation of the 

 future brain, and soon presents a pointed process which is the rudi- 

 ment of the spinal cord. These parts at first appearing granulous 

 are subsequently found to consist of vesicles. 



Thus the central portion of the nei'vous system is not originally 

 a fluid contained within a tube, but developes itself in a solid 

 form before any other part. The central portion of the nervous 

 system sometimes attains a considerable degree of developement, 

 although it be exceedingly minute ; thus an instance has been met 

 with in which the developement of this part had reached a stage 

 scarcely inferior to that in another instance, in which the corre- 

 sponding part measured more than ten times the length. 



There does not occur in the mammiferous ovum any such jiheno- 

 menon as the "splitting" of a membrane into the so-called "se- 

 rous, vascular, and mucous lamina?." Rathke had already found 

 that parts previously supposed by Baer and others to be formed by 

 tiie so-called " germinal membrane," really originate independently 

 of it : these parts are the ribs, pelvic bones, and the muscles of the 

 thorax and abdomen, which according to Rathke arise in a part j)ro- 

 ceeding out of the " primitive trace" itself. Ileichert liad ])reviously 

 discovered that the part originating the lower jaw and hyoid l)one 

 " grows out of the primitive trace." The author beginning with an 

 earlier ])criod goes farther than these observers, and shows that 

 the so-called " primitive trace" it«elf does not arise in the substance 

 of a meml)rane, Imt jiresents a comparatively advanced stage of the 

 object above described as the true germ. Hence the author suggests, 

 there is no structure entitled to be denouiinatctl the " germinul 

 membrane." 



