90 REVIEW. 



at a time, too, when it is, in no sense whatever, morphologically 

 equivalent to a Protozoon, but has taken on all the characters of 

 a triploblastic Enterozoon. I may mention that this active 

 ingestion of yolk by embryonic cells was first observed by 

 Professor Ray Lankester in the Cephalopod's egg. 



The mesoblastic cells, the origin of which was described above, 

 have, in this stage, wandered as far forward as the optic fossae. 

 Amongst them, and evidently taking part in the formation of 

 the middle layer, is a number of globular sliarply-contoured 

 bodies, of variable size, filled with finely granular protoplasm, 

 and containing vacuoles and nucleus-hke structures. These are 

 called by Reichenbach the secondare/ mexoderm cells, to distin- 

 guish them from the primary mesoderm cells formed in the 

 process of closure of the blastopore. The origin of the former 

 will be better discussed under the sixth developmental stage, 

 when their true relations are more clearly seen than at present. 



In the fifth stage (PI. VI, fig. 5) a depression (Oe.) has made 

 its appearance in the medullary groove, about half way between 

 the emarginate anterior boundary of the procephalic lobes and 

 the abdominal process ; this is the commencement of the ecto- 

 dermal invagination which afterwards becomes the foregut 

 [stomodceiim, Ray Lankester). The thoracico-abdominal pro- 

 cess (a) has taken on a roughly pentagonal shape, and in its 

 centre is a shallow depression {an.), the commencement of the 

 hindgut {proctodcEiim) . The mouth and anus, therefore, appear 

 contemporaneously, or nearly so. 



The blastopore appears, at first sight, to have wholly disap- 

 peared, but a close examination of favorable specimens shows 

 that the cells immediately posterior to the thoracico-abdominal 

 process have a peculiar arrangement. Their long axes are turned 

 towards the observer, and they are so disposed as to form a sort 

 of funnel-like depression (g). This depression marks the place 

 of closure of the blastopore. 



The first of the appendages to make their appearance — the 

 mandibles — are seen at this stage as a pair of elevations [Md.), 

 well-defined posteriorly but shading off into the general surface of 

 the blastoderm in front, situated one on either side of the middle 

 line, just in front of the thoracico-abdominal process. Behind 

 and at the sides of the same process, and separated from it by a 

 furrow, has arisen an indistinct semilunar fold (u), the rudiment 

 of the carapace. 



A longitudinal section at this stage shows the complete separa- 

 tion which has taken place between the ectoderm and endoderm, 

 the archenteron being now a completely closed cavity — the 

 midgut. It is evident, too, that the anus is not formed until 

 after the closure of the blastopore, for, while there is u distinct 



