ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
17 
right and left — from the margins of the head-process, while the other 
and principal portion originates from the posterior end and from the 
margins of the primitive streak. 
The memoir becomes more interesting as the author proceeds to 
discuss how the formation of mesoderm in Amniota is to be derived 
from that of Anamnia, and that again from such a mode as Amphioxus 
exhibits. Eabl has a good deal to say about the yolk, and expounds 
most lucidly his theory of its repeated acquisition and loss throughout 
the history of Vertebrates. The eggs of Amphioxus and the Cyclosto- 
mata are primarily poor in yolk ; the poverty is also true of Ganoids 
and Amphibians, but here it is secondary ; while in placental mammals 
it is tertiary. Similarly the eggs of Elasmobranchs are primarily rich 
in yolk, while those of Teleosteans, Sauropsida, and Monotremes are only 
secondarily so. Having discussed the yolk, the author seeks to connect 
the various forms of gastrulation, and points out in so doing that the 
yolk of the Protamniota ought naturally to be situated where the 
principal mass lay in their amphibian ancestors, viz. in front of and 
ventral to the blastopore. He is led to the opinion, so often expressed 
that the primitive groove of Amniota represents the blastopore, and 
the primitive streak its coalesced margins. The dorsal margin of the 
blastopore in Amphioxus, Cyclostomata, and Amphibia, the posterior 
margin of the blastopore in Elasmobranchs, Ganoids, and Teleosteans, 
and the anterior end of the primitive groove of Amniota are all homo- 
logous. The same is true of the ventral margin of the blastopore in 
Amphioxus, Cyclostomata, and Amphibians, the anterior margin in 
Elasmobranchs, Ganoids, and Teleosteans, and the posterior end of the 
primitive groove of Amniota. Eabl supports this conclusion by argu- 
ments drawn from the nature of the segmentation, the formation of the 
mesoderm, the origin of the neurenteric canal, and the formation of the 
blood. In the course of his argument he urges that the metamerism 
of the Vertebrate body has its origin always from the gastral, never 
from the peristomial mesoderm, and also that a vertebral segment 
always arises behind a vertebral segment, the first one appearing with- 
out exception behind the position at which the auditory vesicle is 
formed. 
Eabl then passes to consider the homology of the mesoderm in the 
Bilateralia. His general conclusion is that in all Invertebrate Bilateralia 
the mesoderm has its origin from two rudiments separated in the median 
line and derived from the endoderm of the blastopore margin. He 
makes an exception, however, on behalf of the Chaetognatha. Having 
gained the above general result, Eabl proceeds to show that a perfect 
homology obtains between the mesoderm of the Invertebrate Bilateralia 
and that of Vertebrates. On questions of detail, he inclines to believe 
that the mesoderm had its phylogenetic origin in two endoderm cells 
symmetrically situated by the margin of the blastopore, and along with 
Hatschek still suggests that the primary function of these cells was 
reproductive. 
II. The Differentiation of the Mesoderm. One general conclusion 
stands out among the rest. Tlie head of Vertebrates is regarded as 
consisting of two portions— an anterior, larger, unsegmented region, and 
a posterior, smaller, segmented part. This is true botli ontogeiietically 
1890. c 
