CLEAVAGE 329 



embryonic period of the organism, and later disappear or remain 

 only as rudiments. 



709. In the dicotyledon embryo a similar embryonic structure 

 is found. The suspensor is a row of cells which forms no 

 part of the young plant. The terminal cell divides into two, 

 four, eight, etc., cells, which at first form a spherical mass. 

 This soon becomes heart-shaped and then the four parts of the 

 dicotyledon embryo may be localized, viz. : I the caulicle, II 

 the epicotyl, and III and IV the cotyledons. 



710. In many of the higher cryptogams the new cells are all 

 divided off from one terminal or apical cell. In some cases a 

 few divisions may occur in segments cut off from the apical 

 cell, but only the apical cell is capable of continued division. 

 In the dicotyledon, cell multiplication takes place only in limited 

 regions of the bud and root-tip and in the cambium. These 

 have already been described (p. 48). (Growth in Mono- 

 cotyledons, see p. 343.) 



711. Types of Cleavage. — The course of segmentation of 

 animal eggs is greatly modified by the quantity of yolk present 

 in the egg, and in this respect animal eggs vary greatly. Some 

 eggs contain little yolk (Ccelenterates, Worms, Echinoderms, 

 the lancelet. Mammals), others have moderate quantities (many 

 Molluscs, some Fishes, Amphibia), while in still others the 

 quantity of yolk may be many times in excess of the protoplasm 

 (Arthropods, Cephalopods, many Fishes, Reptiles and Birds). 

 The effect of the yolk is to retard or even completely inhibit 

 division. The simplest type of development occurs, therefore, 

 in eggs with little yolk, and as an example of this type we will 

 take the egg of the lancelet. 



712. The Blastula. — The first cleavage plane of the egg of 

 the lancelet is vertical, the second is also vertical and at right 

 angle to the first. The third is horizontal and cuts the four seg- 

 ments slightly above the centres, so that the four lower cells are 

 slightly larger than the four upper. This is due to the slightly 



