INTRODUCTION 



FIG. 3. BLASTOSPHERE. 



BD, blastoderm ; FH, segmentation 

 cavity. 



derm. Consisting primarily of a single layer of cells, the blasto- 

 derm later on becomes two-layered, and then three-layered. From 

 the relative position of these, they are spoken of respectively as 

 the outer, middle, and inner 

 germinal layers, or as ecto- 

 derm (epiblast), mesoderm 

 (mesoblasf), and endoderm 

 (hypoblasf). 



The mode of distribution 

 of the yolk-particles in the 

 ovum, and an increase in their 

 amount, result in certain 

 modifications of the primitive 

 form of segmentation as de- 

 scribed above. Yolk is an 

 inert substance, and its pre- 

 sence tends to hinder or even 

 entirely to prevent segmenta- 

 tion in those parts of the 

 ovum in which it is abundant. 

 When the whole ovum undergoes division, the segmentation is 

 known as entire or holoblastic ; when division is restricted to part 

 of the ovum onlv the segmentation is said to be partial or mero- 



blastic 1 (Fig. 4). 



The question as to the origin of the 

 germinal layers, on account of its im- 

 portant signification, is one of the most 

 burning problems in morphology, and as 

 yet we cannot arrive at any full and 

 satisfactory conclusion on the subject. 

 It may, ho \vever. be stated that in all 

 Vertebrates the blastosphere passes or 

 did so in earlier times into a stage 

 called the gastrula, which is retained 

 in an unmodified form only in the lowest 

 Vertebrate (Amphioxus, cf. p. 14). The 

 Bla, blastoderm ; Do, yolk, gastrula is derived primitively from the 



blastula by the walls of the latter 



(Fig. 3) becoming pushed in or invaginated at one part, thus 

 giving rise to a double-walled sac (Fig. 5). The outer wall 

 then represents the ectoderm, which serves as an organ of 

 protection and sensation, while the inner, or endoderm, encloses 



1 In holoblastic segmentation the resulting cells are approximately equal in 

 the Lancelet and in Mammals (with the exception of Monotremes and some 

 Marsupials) ; and unequal in the Cyclostomes, Sturgeon, Lepidosteus, Dipnoans, 

 and nearly all Amphibians, the segmentation sometimes approaching the mero- 

 blastic type. In Elasmobranchs, Teleosts, Reptiles, Birds, and Monotremes 

 the segmentation is from the first meroblastic and discoid, i.e., restricted to the 

 upper pole of the ovum (Fig. 4). 



B* 



J)0 



FIG. 4. DIAGRAM OF A MERO- 

 BLASTIC OOSPERM WITH 

 DISCOID SEGMENTATION-. 



