No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 3 



which produced such a voluminous discussion (see Part I, list 

 of literature) was in regard to the real nature of the curious 

 little bivalves, which were often found filling the external gills 

 of adult Unios and Anodontas. Were they parasites or were 

 they the young of the animals which they burdened with their 

 incredible numbers? Rathke (No. 7, 1792) and Jacobson 

 (No. 3, 1828) held that they were parasites of a different 

 genus, and proposed for them the name of Glochidiiim para- 

 siticum. The action of the Paris Academy of Sciences, which 

 appointed a commission (No. 4) to inquire into the foundations 

 of this curious view, is sufificient proof of the lively interest 

 which the question excited. It would take me too far to 

 follow the discussion in detail; it will be sufficient to say that 

 Carus (No. 5, 1832) gave the final blow to the glochidium 

 theory in a paper which is remarkable both for accurate 

 observation and logical argument. Indeed, but little advance 

 was made beyond the facts established in this paper prior to 

 Flemming's work (Nos. 12 to 14). Contributions to the sub- 

 ject had been made in the meantime by Quartrefages (Nos. 23 

 and 24), Leuckart (No. 19), O. Schmidt (No. 20), Forel (No. 

 21), and van Ihering (No. 22). In 1866 Leydig (No. 20d) 

 made the valuable discovery that the glochidium larva finishes 

 its development as a parasite on fish. 



Flemming's principal paper appeared in 1875 (No. 13). He 

 gave some good figures of the maturation and early segmenta- 

 tion stages of the Qg%, and of the later larval stages; but he 

 remained in the dark as to the passage from the one to the 

 other, and hence came to no definite conclusion as to the origin 

 and formation of the germinal layers. Rabl's paper (No. 25) 

 appeared in 1876, and is in some ways a distinct advance on 

 Flemming's. He endeavored to prove that the germ-layer 

 theory was applicable to the Unionidae " wie bei alien Meta- 

 zoen." Although he was undoubtedly right in saying that the 

 " Vorderwulst " of Flemming was entodermic, and that the 

 mesoderm arose from two teloblasts, yet he was just as un- 

 doubtedly wrong in his account of the origin of the entoderm 

 as well as of the teloblasts of the mesoderm. According to 

 Rabl, the entoderm arose as an invagination of large cells in 



