200 THE EVOLUTION OF IkLA^N. 



Among the many and various kenogenetic or vitiated 

 forms of esrix-cleavaofe and orasfcrulation, I aerain distinomish 

 three different chief forms : 1. Unequal cleavage (segment 

 tatio incequalis, Plate II. Fig. 7-17) ; 2. Discoidal cleavage 

 (segmentatio discoidalis, Plate III. Fig. 18-24) ; and 3. 

 Surface cleavage (segmentatio superjicialis, Plate III. Fig. 

 25-80). Unequal cleavage results in a Hood-gastrula 

 (Amphigastrula, Plate II. Fig. 11 and 17); discoidal cleavage 

 results in a Disc-gastrula (Dlscogastrula, Plate III. Fig. 24); 

 surface cleavage results in a Bladder-gastrula (Perigastrula, 

 Plate III. Fig. 29). The last form does not occur among 

 Vertebrates, with which we are now specially concerned ; 

 it is, on the contrary, the usual form among Articulated 

 Animals (Spiders, Crabs, Insects, etc.). In Mammals and 

 Amphibia the cleavage is unequal, and the Gastrula is a 

 Hood-gastrula ; this is equally true of the Ganoid fish and 

 the Round-mouths (Lampreys and Hagfishes). On the other 

 hand, in most Fishes, and in all Reptiles and Birds, we find 

 the discoid form of cleavage, and a Disc-gastrula. (Cf. 

 Table III.) 



As Man is a true Mammal, and as human germination 

 is entirely similar to that of other Mammals, the cleavage 

 in his case also is unequal, and results in the formation of a 

 Hood-gastrula ( A mphi gastrula, Plate II. Fig. 12-17). But 

 it is peculiarly difficult to investigate the first incidents in 

 the ecffj-cleavao^e and orastrulation of Mammals. It is true 

 that more than thirty years ago the anatomist Bischoff, of 

 Munich, laid a foundation for this work in two books, which 

 he published, on the germ-history of the Rabbit (1842), and 

 on that of the Dog (1845); and that these were afterwards 

 followed by two equally careful studies of the gernnuation 



