88 REVIEW. 



A circular thickening of the ectoderm has taken place in each 

 of the procephalic processes, forming a pair of discs (Kopf- 

 scheiben, k), the centres of which are depressed in such a 

 manner as to form two small rounded pits (v). The cells of 

 these, which may be called the optic fossse^ are afterwards con- 

 verted into the optic ganglia, and probably into the retinal 

 elements, so that, as in Cephalopods, the organ of sight origi- 

 nates as an invagination of the epiblast. No previous observer 

 seems to have noticed these structures : both Eathke and 

 LerebouUet describe the eyes as originating after the antennae 

 and mandibles in the form of ridges ; in other words they over- 

 looked the first appearance of the eye itself, and saw only that 

 of the ophthalmic peduncle. 



A point of great interest, brought out by section of embryos 

 in the first three stages, is the origin of the mesoderm, which is 

 conclusively shown to arise as a direct product of the eudoderm. 

 The cells of the ectoderm are, as has already been mentioned, 

 regularly columnar, passing insensibly at the edge of the em- 

 bryonic area into the ordinary pavement cells of the blastoderm ; 

 they also rarely contain more than one nucleus, and evidently 

 proliferate very slowly. The eudoderm cells, on the other hand, 

 are very variable in form, and frequently contain two or even 

 three nuclei. These characters, an evident sign of active multi- 

 plication, are most marked on the anterior steep edge of the 

 blastopore, the boundary region between ectoderm and eudo- 

 derm ; here the multiplication of the cells (see fig. 4, woodcut) is 

 very evident; some are seen to be dividing longitudinally, form- 

 ing cells similar to themselves; others are dividing transversely, 

 the pinched-off inner moieties assuming the form of mesoderm 

 cells. These latter do not, as yet, form a continuous tissue, but 

 lie scattered about the deutoplasm ; they vary a good deal both 

 in form and size. This may be due either to pressure or to 

 amoeboid movements ; very probably .the latter, as they are already 

 found a considerable distance from their place of origin. 



It becomes now of great importance to decide the question as 

 to the real nature of this boundary region : ought it to be considered 

 as belonging to the ectoderm or to the eudoderm ? A transverse 

 section through the blastopore in the third stage shows in a very 

 instructive manner how the diminution in diameter of the gastrula- 

 mouth is brought about, namely, by the active proliferation and 

 consequent ingrowth towards the middle line of the cells forming 

 its lateral edges. The cells as they grow inward constantly undergo 

 division, and thus by one and the same process the blastopore is 

 closed and the mesoderm added to. An examination of later stages 

 shows that, by this process of closure , the whole of the cells forming 

 the front slope of the biastopori.' come to stnnd at the edge of 



