I08 BIOLOGICAL LECTURES. 



bryologist of hollow organs that arise either by invagination 

 or by delamination ; of paired organs that arise from either 

 single or paired foundations, and vice versa. No one is dis- 

 posed to question the homology of the spinal cord of a teleost 

 with that of a shark on the ground that the one arises as a 

 solid cord, the other as an infolded tube. The stomodasum 

 of LopadorhyiicJiiLS is undoubtedly homologous with that of 

 Lumbriciis, though the one appears as a paired, the other as 

 a single median structure. The ventral nerve-cord of Poly- 

 gordiits is certainly homologous with that of Liinibricus, though 

 the former appears as a median unpaired thickening of ecto- 

 blast while the latter arises by the concrescence of two widely 

 separated halves. A very striking example of this kind is 

 afforded by the development of Balanoglossiis. In B. Kowa- 

 levskii the third pair of body-cavities arise as archenteric 

 pouches (Bateson); in the Bahaman form, as shown by Mor- 

 gan,^ these same cavities are formed, not as pouches, but by 

 the aggregation of scattered mesenchyme cells of unknown 

 origin. Such cases show that the particular modus operandi 

 by which a structure arises in the larva gives in itself no cer- 

 tain standard of morphological value. Position is equally un- 

 trustworthy, for in numberless cases homologous parts differ 

 in this respect, for example, the heart of mammals and birds, 

 the mesoblast-bands of leeches and oligochaetes (which are at 

 right angles to those of many polychaetes), the nerve-cords of 

 Polygordius and Liivibricus, as already cited. Any embry- 

 ologist could cite scores of cases of homologous parts that 

 arise in some cases at or near their final position, in other 

 cases far removed from it, so that very extensive growth is 

 necessary to transport them thither. 



Let us now examine briefly the gastrular stages, that is, the 

 origin of parts with respect to the germ-layers. Until very 

 recently the primary germ-layers have been — and with jus- 

 tice — regarded as the most constant and trustworthy basis of 

 comparison ; and so constant is the connection between them 

 and the adult organs {e.g., between ectoblast and nervous sys- 

 tem) that it is difficult not to regard this connection as a fixed 



^ Journ. Morph., IX, 1894. 



