I6i6 A MANUAL OF ANATOMY 



The blastoderm is the wall of the blastodermic vesicle, this vesicle repre- 

 senting the fertilized ovum after segmentation has been completed, and after 

 a cavity, called the segmentation-cavity or blastocoele, has been formed 

 within the morula or mulberry-like mass of cells which results from the 

 segmentation of the fertilized ovum. The blastoderm is at first unilaminar, 

 and is formed by the outer cells of the morula, which spread out and form 

 the enveloping layer. This layer represents the primitive ectoderm, and, 

 being extra-embryonic, it gives rise to the foetal part of the placenta and the 

 foetal membranes. Meanwhile, during the formation of the cavity of the 

 blastodermic vesicle, the remaining cells of the morula become collected into 

 a round heap, called the inner cell-mass, which is situated at the upper em- 

 bryonic, or germinal pole of the blastoderm. The cells of the inner cell-mass 

 which lie next the segmentation-cavity, spread out and ultimately form a 

 layer, which lines the primitive ectoderm ftr enveloping layer. This lamina 

 represents the primitive entoderm, and the blastoderm is now bilaminar. 

 !Prior to the completion of this entodermic lamina, certain changes take 

 place within the remainder of the inner cell-mass. Some of its cells give 



Neural Tube 



Ectoderm ^^^^^^S-. Myocoel 



Mesoderm^ >^^^!^^^ -' Lateral Mesoderm 



/ Somatopleure 



Coelom , ^jg7%:S^Se )^<%1l^^ ^S/^^I^MbSS^^^ '' / Coelom 



i 

 Dorsal Aorta ,'' • Dorsal Aorta Splanchnopieure 



Entoderm Notochord 



Fig. 670. — The Germinal Layers. 



rise to the embryonic bud or disc, which is the rudiment of the embryonic body. 

 In others vacuoles appear, which coalesce and form a cavity within the upper 

 part of the inner cell-mass, beneath Rauber's layer, called the amniotic cavity. 

 The floor of this cavity is formed by cells derived from the embryonic bud. 

 This layer of cells represents the embryonic ectoderm, which becomes con- 

 tinuous with the extra-embryonic ectoderm forming the enveloping layer. 

 Up to this stage the blastoderm has remained bilaminar. 



Formation of the Mesoderm. — On either side of the primitive streak and its 

 head process, cell-proliferation takes place, and the new cells thus formed 

 gradually extend, on either side, into the blastoderm, insinuating themselves 

 between its two layers — ectodermic and entodermic. In this manner a third 

 or intermediate layer is formed, which represents the primitive mesoderm, 

 this being secondary as regards development. It receives its cells chiefly 

 from the ectoderm of the primitive streak and its head-process, but it is rein- 

 forced by cells derived from the subjacent entoderm. 



The blastoderm is now trilaminar, being composed, from without inwards, 

 of ectoderm, mesoderm, and entoderm. 



Characters of the Germinal Layers. — ^The cells of the ectoderm 



are of an epithelial nature. In the region of the embryonic or 

 germinal pole of the blastoderm they are somewhat columnar, but 

 elsewhere they are flattened. The cells of the entoderm are also of 

 an epithelial nature, and are all flattened in the first instance ; but 

 subsequently they become columnar. The cells of the mesoderm 

 are at first separated by intercellular vacuoles, containing fluid, and 

 they are branched. 



