1154 



EMBRYOLOGY. 



pr- 



FIG. 696. Embryo of a rabbit of 

 eight days. (After Kolliker.) arg. 

 Embryoiiic area. pr. Primitive 

 streak. 



cell-walls) ; the inner layer assumes the form of a prismatic epithelium, and is 

 named the cytoblast (Fig. 695). These two layers form the ectoplacenta or 

 chorion, and entirely replace the lining epithelium of the uterus where the blasto- 

 dermic vesicle comes into contact with it. According to Van Beneden, the cells 

 of the inner mass partly undergo atrophy (Fig. 694), giving rise to a cavity, 

 limited above by the cytoblast and below by a layer of cells, which constitutes the 

 primitive upper layer of the embryo, the epiblast or ectoderm, and which is con- 

 tinuous peripherally with the cytoblast. The cavity thus formed is the primitive 



amniotic cavity, and becomes the permanent amniotic 

 cavity in man and monkeys, and in some of the bats 

 (Fig. 695). It will thus be seen that from the inner 

 mass of cells two layers are formed an outer of pris- 

 matic cells, the epiblast or ectoderm, and an inner of 

 flattened cells, the hypoblast or entoderm and this 

 double layer constitutes the blastodermic membrane, 

 which at this stage is bilaminar. 1 



3. Formation of the Mesoblast. At first the area 

 of the blastodermic membrane assumes the form of a 

 small disk, the germinal disk or germinal area. This 

 disk becomes oval in shape, with its more pointed end 

 situated posteriorly. In it the first traces of the 

 embryo are seen as a faint streak, the primitive 

 streak (Fig. 696), which makes its appearance at 

 the posterior or narrow end of the oval disk and 

 from there gradually extends forward. The epiblast covering the primitive streak 

 becomes indented by a groove, the primitive groove, the anterior end of which 



communicates through a canal 

 with the yolk-sac, forming what is 

 termed the blastopore. The prim- 

 itive streak results from a multi- 

 plication of the cells of the epi- 

 blast, so that it becomes thickened 

 and grows downward toward the 

 hypoblast, which also undergoes 

 proliferation. Together they form 

 a thick cellular column, in which 

 it is no longer possible to distin- 

 guish the epiblastic from the hypo- 

 blastic cells. From the sides of 

 this column a layer of cells grows 

 out between the epiblast and hypo- 

 blast, having been derived partly 

 from both ; this layer constitutes 

 the mesoblast or mesoderm. 



In this way the blastodermic 

 membrane comes to consist of three 

 layers, and is now known as the 

 trilaminar blastoderm. Each layer 

 has distinctive characters, the outer 

 and inner presenting the ap- 

 pearance of epithelial cells, while 

 the middle consists of a mass of 

 branched cells without any definite arrangement. The external is termed the 

 epiblast, or ectoderm; the internal the liypoblast, or entoderm; and the middle, 

 the mesoblast, or mesoderm (Fig. 698). 



1 Consult, in this connection, articles by Van Beneden and Kollraann, Analomischer Anzeiger, 

 1899 and 1900. 



FIG. 697. Embryonic area of the ovum of a rabbit at the 

 seventh day. ay. Embryonic area, o o. Region of the blas- 

 todermic vesicle immediately surrounding the embryonic 

 area. pr. Primitive streak, rf. Medullary groove. (From 

 Kolliker.) 



