INTRODUCTION 



FIG. 3. BLASTOSPHERK. 



substance, and its presence tends to hinder or even entirely to 

 prevent segmentation 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 

 only, the segmentation is said 

 to be partial or meroblastic 1 

 (Fig. 4). 



The question as to the origin 

 of the germinal layers, on ac- 

 count of its important significa- 

 tion, 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, how- 

 ever, be affirmed with certainty 

 that in all Vertebrates the 



blastosphere passes Or did SO BDt blastoderm ; FH, segmentation 



in earlier times into a stage cavity. 



called the gastrula. One 



must imagine this form as being derived primitively from the 

 blastula by supposing that the walls of the latter (Fig. 3) became 

 pushed in or invaginated at one part, thus giving rise to a double- 



walled sac (Fig. 5) The outer wall then 

 represents the epiblast, which functions 

 as an organ of protection and sensation, 

 while the inner, or hypoblast, encloses 

 a central space, the primitive intestinal 

 cavity (archenteron), and represents the 

 assimilating and digestive primary ali- 

 mentary canal. The opening of the 

 latter to the exterior, where the two 

 germinal layers are continuous, represents 

 FIG. 4. DIAGRAM OF A MER- the primitive mouth or blastopore 



o* 5) 



Q^ of the epiUat arige kter the 



Bla, blastoderm; Do, yolk, epidermis and its derivatives, the entire 



nervous system, the sensory cells, the 



crystalline lens of the eye, and the oral and anal involutions 

 (stomodceum and proctodceuni). In an early stage the hypoblast 

 gives rise to an axial rod, the notocliord (see p. 9), and eventually 

 to the epithelium of the greater part of the alimentary canal 



1 In holoblastic segmentation the resulting cells are approximately equal in 

 the Lancelet and in Mammals (with the exception of Monotremes) ; and unequal 

 in the Cyclostomes, Sturgeon, Lepidosteus, Ceratodus, and nearly all Amphibians, 

 the segmentation sometimes approaching the meroblastic type. In Elasmo- 

 branchs, Teleosts, Reptiles, Birds, and Monotremes the segmentation is meroblastic 

 and discoid, i.e., restricted to the upper pole of the ovum (Fig. 4). 



ffla, 



OBLASTIC OOSPERM WITH 



DISCOID SEGMENTATION. 



