82 The Microscope. 



were reconciled. In several places he hinted at the explana- 

 tion, and last year his pupil, Mr. Adam Sedgwick, of Cambridge, 

 formulated the results. As far as we are at present concerned, 



Fig. 4. Diagram of regular Invagination, b 1, blastopore; e, epiblast; h, hy- 

 poblast; se, archenteron ; sc, segmentation cavity. 



Fig. 5. Diagram of epibolic gastrulation : h, solid hypoblast, the smaller 

 cells are epiblastic. 



the conclusion is, that the blastopore is homologous with both 

 mouth and anus, and that when it completely closes, these 

 openings subsequently reappear in the line and within the lim- 

 its of the primary blastopore. 



This explanation doubtless contains the germs of the truth, 

 and indeed it explains an enormous number of facts. There 

 are still others which are yet to be brought into sympathy with 

 it, but how is as yet uncertain. To mention only one, we would 

 recall the mouth of the vertebrates. Is this homologous with 

 that of the invertebrates, or is it a superadded feature ? 



Let us now consider the third of the embryonic layers, the 

 mesoblast or mesoderm, as it occurs and is developed in the 

 majority of animals. In the adults of all except some of the 

 ccelenterates, and possibly some sponges, this layer lies between 

 the hypoblast and the epiblast, and in the higher groups plays 

 a very important part in the building up of the embryo. In the 

 vertebrate, for instance, it forms the lower portion of the skin, 

 the muscles, bones, heart, blood, connective tissue, excretory 

 and reproductive organs ; in fact, everything between the outer 

 layer of the skin and the epithelium of the alimentary tract 

 and its appendages, except the nervous system and one or two 

 other structures which it is not necessary to enumerate here. 



In the typical egg it has the development shown in the cut 

 of the egg of Amphioxus, copied from Kowalewsky's memoir. 

 The figure represents a transverse section, and shows but two 



