INTRODUCTION. xi 



result, known ns the blastopore. Tliis blastopore in most forms completely closes ; but 

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

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

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

 those of the einblast. Tiiis 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 inecjuality occurs, until in some forms the hypoblast is invaginated as a few 

 cells, or even a single cell, and the archenteron does not appear 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 known as delamination. A general idea of the ]>rocess may 

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

 segmented off to form the hypoblast. 



In the gastrula we have two of the so-called germinal 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- 

 blastic. 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 Imd 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 



-^ ^ formation <jf the, germ layers 



of the same figure, and the walls of these pouches are the (but little modiflea from that 



TCI !• 1 occurring in f*t'ripafns); on the 



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



-, . ., . 1.111, later stage; h, blastopore; c, 



and an enumeration of the organs into wluch they develop. cceiom; </, mesobiastic pouch; 



. J. , 1 1 i 1 ii »• £ e, epiblast; h, hypoblast; m. 



After the formation oi the mesoblast and the separation oi mesenteron; o.somatopiure;/), 



!• ^1 ' 1 , xi 1 1 1 i* -i. • spianchnoplure; 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 comideteiy closed, but in some forms it 

 remains open, forming the mouth or the vent, and in Perijxttus it closes in the middle, 

 leaving both oral and anal openings 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 com])lete alimentary 

 tract. From this method of form.ation of the anterior and posterior jiarts of the 

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



