7^6 
The An 
ralist. 
[September, 
The Gastriila theory is that all metazoa have 
herited stage of development, which follows immediately after 
the diaderm ; this stage is characterized by there being an outer 
ectodermal sack with a perforation, to the edge of which is 
attached the entoderm, which forms a closed inner sack, the 
archenteron. 
The term gastrula was introduced by Haeckel, and is now 
universally used by embryologists. The discovery of the im- 
portance of the gastrula is due to the brilliant researches of 
Kowalewski on various invertebrates, including Amphioxus, then 
supposed to be a vertebrate. Haeckel then seized upon the idea 
of the gastrula, and wrote an essay, 21, upon it, which from its 
brilliant style attracted much notice, and did much to direct 
attention to the important discovery of Kowalewski. Although 
Haeckel indulged his fantasy unduly, and was misled into 
speculations which are now unheeded and almost forgotten, he 
did great good by starting the interest of zoologists in the right 
direction. By a remarkable coincidence Lankester published an 
essay, j/, of a purport very similar to Haeckel's, at about the same 
time. A great deal remains to be done before the gastrula 
theory of evolution can be fully established, for there are many 
facts not brought into accord with the theory in its present form. 
The gastrula, like the dia- 
derm, varies greatly, the chief 
modifications depending on 
the amount of yolk present ; 
this is illustrated by the ac- 
companying diagrams, (Fig. 
20). The mesoderm is in- 
tentionally omitted: A cor- 
responds to such a larva as 
Fig. 19; the difference in 
size between the two sets of 
cells is slight but evident. 
In B the difference is more marked, and fairly represents a gas- 
trula of Amphioxus. In C the difference is very great, and 
corresponds to that observed in certain gastropod larv^. In 
