GERMINAL LAYERS OF THE EMBRYO. <«od 
that of Nephelis amongst the Discophora is not capable of 
being reduced to the invaginate type. 
A 
Pig. 1 .■^Segmentation and Formation of the Blastoderm in Chelifer. (After 
Metscbnikoff.) In A the ovum is divided into a number of separate 
segments. In B a number of small eells have appeared {bL), whieh 
form a blastoderm enveloping the large yolk spheres. In C the blasto- 
derm has beeome divided into two layers. 
The development of almost all the parasitic groups, i.e, the 
Trematoda,the Cestoda, the Acanthocephala, and the Lingua- 
tulida, and also of the Tardigrada, Pycnogonida, and other 
minor groups, is too imperfectly known to be classed with 
either the delaminate or invaginate types. 
It will, I think, be conceded on all sides that, if any of the 
ontogenetic processes by which a gastrula form is reached are 
repetitions of the process by which a single two-layered gas- 
trula was actually developed from a compound Protozoon, 
these processes are most probably either invagination or de- 
lamination. 
The much disputed questions which have been raised about 
the gastrula and planula theories, originally put forward by 
Haeckel and Lankester, resolve themselves then into the 
single question, whether any, and if so which, of the onto- 
genetic processes by which the gastrula is formed are repe- 
titions of the phylogenetic origin of the gastrula. 
It is very difficult to bring forward arguments of a con- 
clusive kind in favour of either of these processes. The 
fact that delaminate and invaginate gastrultc are in several 
