INTRODUCTION. xi 



result, known as the blastopore. This blastopore in most forms completely closes ; but 

 to this we will return again. The hollow, which we have mentioned above as form- 

 ing primarily the digestive cavity, is known as the archenteron or primitive stomach, 

 and the hypoblastic cells which form its boundary are almost invariably larger than 

 those of the epiblast. This is true of all gastrulas, even those where the segmentation 

 is regular, and the reason is not difficult to find. The external cells have to embrace a 

 greater superficial extent than the internal ones, and hence the layer becomes thinner 

 and the resulting cells smaller. In other cases the segmentation is irregular and then 

 a greater inequality occurs, until in some forms the hypoblast is invaginated as a few 

 cells, or even a single cell, and the archenteron does not apj)ear until a later date, when 

 it is hollowed out of the hypoblastic cells. A greatly different mode of forming the 

 gastrula is by what is knov/n as delamination. A general idea of the process may 

 be obtained by saying that the inner ends of the cells of the blastula (Fig. 11., G) are 

 segmented off to form the hypoblast. 



In the gastrula we have two of the so-called geiminal layers, the epiblast and the 

 hypoblast ; in all animals except some of the coelenterates and the Dicyemids, a third 

 layer, the mesoblast or mesoderm, occurs, hence these are 

 known as triploblastic animals, in contradistinction to those 

 with only hypoblast and epiblast, which are called diplo- 

 blastie. We will not enter into a discussion of the many 

 different ways in which the mesoblast arises, but will merely 

 indicate what is apparently the typical method, which in 

 reality exists unmodified in but very few animals. From 

 the hypoblast, pouches bud off on either side, as shown on 

 the left of our figure. These pouches eventually become 

 separated from the archenteron, as shown on the right side fig. v.— Diagram illustrating the 



J^ ' '^ formation of the germ layers 



of the same figure, and the walls of these pouches are the (but nttie modified from that 



° T ^ 1 ,.1 occurring in Peripatus); on the 



mesoblast. Now we are ready for the names of these parts right an earlier, on the left a 



. i-iiii later stage; b, blastopore; e, 



and an enumeration of the organs into which they develop. coeiom; S, mesobiastio pouch; 

 After the formation of the mesoblast and the separation of mesenteroii: o,'somatopiure;pJ 



J. .^ , . ii_ 1 1 1 i- -4. ■ splanohnoplure: s, segmenta- 



a portion of the archenteron, the hypoblastic cavity is tion cavity. 



known as the mesenteron, from the fact that its lining cells 



form the epithelium of the middle portion of the digestive tract. From other pouches 



and outgrowths of the mesenteron, formed at a later date, other organs arise. Among 



these we may mention the liver, the lungs of vertebrates, the endostyle of tunicates, 



the thyroid and thymus glands, pancreas, spleen, and the notochord. 



The epiblast, as we have seen, gives rise to the outer layer of the skin. This is 

 not the whole of the list of its derivatives, for we must here include the nervous sys- 

 tem and the organs of sense, dermal glands, teeth, membrane-bones, etc. As we have 

 said, the blastopore almost always becomes completely closed, but in some forms it 

 remains open, forming th^ mouth or the vent, and in Peripatus it closes in the middle, 

 leaving both oral and anal openhigs at its extremities. In these forms where it 

 becomes completely closed, the mesenteron is entirely separated from the external 

 world, and communication has to be again opened with the exterior. This is accom- 

 plished by inpushings of the epiblast at the extremities of the body. These ingrowths 

 finally meet and unite with the hypoblast, and thus form the complete alimentary 

 tract. From this method of formation of the anterior and posterior parts of the 

 digestive canal, it follows that certain internal organs, as the oesophagus and intes- 



