1891.] Embryology. 57 
Each of the sections ends with a general part, in which the authors 
bring together the essential points of the papers on the subject, and 
give their interpretation of the meaning of the facts. 
The first section, on the Sponges, is admirable, and a much better- 
balanced presentation than that given by Balfour, who has unduly 
emphasized the amphiblastula larva. They believe the Sponge is a dis- 
tinct phylum, only connected with the other Metazoa in its earliest 
history,—‘‘ nur an seiner Wurzel.’’ The group has a true blastula and 
gastrula stage, but believing the polyp to represent the ancestral 
Ccelenterate, they do not think the Sponges have any affinity with the 
latter group. For the Ccelenterates they adhere to the old views that the 
fixed Hydroid represents the ancestral form, and the Medusæ a higher, 
more specialized, and later form; and they reject the free-swimming 
jelly-fish as an ancestral form, as held by Claus, Brooks, and Vogt. The 
authors are inclined to believe in the gastrula as the ancestral form of 
the Cnidaria, and think asecondary change has come into the ontogeny 
“ because the typical larval form of the Cnidaria is the planula. It 
is probable that the transition of the free-swimming, gastrula-like 
ancestor was changed into the fixed polyp form through a creeping 
stage, which the creeping planule of many forms now repeat in their 
ontogeny.”’ : 
The Ctenophors have many general points of agreement with other 
Ccelenterates, but recent work (Hertwig, Lang, Hatschek) goes to show 
that the Ctenophors have had an independent origin. They show no 
evidence of a polyp ancestry in the ontogeny, and the formation of 
organs shows they have no close relationship to the Meduse. To sum 
it all up, the Ctenophors represent an independent phylum of the animal 
kingdom, and only at the base unite with the Cnidaria. 
The Turbellarians show many common characteristics in segmenta- 
tion and gastrulation with the Ctenophors, but if these two groups 
came from the same root, each has become so changed that only general 
resemblances are possible. 
The Trematodes go back to Turbellarian-like flatworms which 
have assumed a parasitic life. The Nemertines, although more highly 
developed, are probably related to the flatworms, and it is not possible 
to separate Nemertines from the Turbellarians in order to place them 
among the segmented worms. Hubrecht’s hypothesis as to the rela- 
tionship between Vertebrates and Nemertines can have only an entirely 
speculative value. The embryology of the Nemathelminthes throws 
no light upon their ancestry, and we cannot determine whether they 
are related to the Nemertines on the one hand, or to the Annelids on 
