30 The Microscope. 



proper name, or rather several names, and as students will run 

 across all of these in their reading, we give them here. The 

 outer of the two layers is variously called epi blast, epiderm, 

 ectoblast and ectoderm. Which of these is preferable is a 

 question. In the publications of English naturalists the first is 

 usually employed, while the Germans generally use the last. 

 By strict laws of priority the term ectoderm should be used, 

 but if we are to have words which indicate exact shades of 

 meaning, epiblast is preferable. The epiblast of the gastrula 

 is something more than the epiderm of the adult, for, as we 

 shall see, from it develops, besides the outer layer of the body, 

 the nervous system and several other structures. 



With the inner layer the same duplication of names is 

 found, and we have hypoblast, hypoderm, entoblast, endoblast, 

 endoderm, and entoderm. The same remarks as to which is 

 preferable which were made in the preceding paragraph, apply 

 here. From the hypoblast of the gastrula is formed not only 

 the endoderm of the adult, but the epithelium of the body cav- 

 ity and of the lungs, liver, and some other glands connected 

 with the digestive and respiratory functions. 



With these points in mind, it will be well to restrict the 

 terms epiblast and hypoblast to the two layers of the gastrula, 

 and apply the names ectoderm and endoderm to the respective 

 derivatives which form the outer layer of the body and the 

 lining of the middle region of the digestive tract. The others, 

 may be regarded as synonyms. 



The extent of the invagination may be rather small, or it 

 may proceed to such an extent that the segmentation cavity is 

 obliterated. With the in-pushing the process of invagination 

 is not completed, but the edges of the cup begin to grow to- 

 gether until its mouth is reduced to a small opening, which 

 either persists through life or later becomes completely closed. 

 This opening is known as the blastopore, and the hollow to 

 which it leads (that bounded by the hypoblast) as the primitive 

 stomach or the archenteron. This stage of the embryo is 

 known as the gastrula, and the process of its formation as gas- 

 trulation. 



This closing together of the lips of the blastopore brings 

 about a difference in the size of the cells of the epiblast and 



