HISTORY OF BLASTODERM. 23 



which has been above alluded to as invaginated ectoderm (primitive groove) as the homologue 

 of part of the entoderm of more typical forms. If this view be correct, many of the difficulties 

 in the way of regarding the aperture of the invagination as the blastopore, and in explaining 

 the differences in the mode of origin of the mesoblast are removed ; but. on the other hand, 

 other difficulties are introduced, and the subject is left by no means clear. 



Another view, which was formerly extensively held, regards the blastopore of the meroblastic 

 vertebrate ovum as bounded by the thickened edge of the bilaminar blastoderm (Haeckel). 

 According to this view, the cavity of the gastrula is entirely filled up by a mass of 

 unsegmented or but partially segmented yolk, which also projects for a considerable distance 

 through the blastopore, forming in fact the great mass of the ovum. The primitive groove 

 is regarded as a linear prolongation of this thickened edge of the blastoderm towards the 

 centre of the blastoderm (Balfour), so that the embryo, which developes in front of the 

 primitive streak, thus comes to have a pseudo-central position in the blastoderm instead 

 of developing altogether from its margin as in the lower vertebrata and in inverte- 

 brates. In conformity with this idea, it may be noted that at the thickened rim of the 

 blastoderm of these meroblastic ova, the two primary layers are continuous with one another 

 as in the primitive streak. In elasmobranchs an intermediate condition is observed, viz., a 

 short groove, the margins of which are freely continuous with the margin of the blastoderm. 

 If, as His and others have described (vide infra), the mesoblast is in part (vascular 

 and connective tissue part) derived from the thickened rim of the blastoderm, this would 

 furnish another point of resemblance between the primitive streak and that margin. 



Ed. v. Beneden has promulgated an entirely different opinion as to the mammalian blasto- 

 pore from those above described. He regards the condition of the ovum, after the completion 

 of segmentation and before the formation of a blastodermic vesicle, as representing the 

 gastrula stage, and looks upon the point where the granular inner mass of cells comes to the 

 surface between the clear cells which form the outer investment as the blastopore (fig. 14, a). 

 In conformity with this view he considers the layer of clear cells to represent the whole of the 

 primitive ectoderm, and the granular inner mass the primitive entcderm. But since all the 

 more recent observations upon early mammalian ova agree in affirming the formation of 

 the three blastodermic layers from the granular inner mass, and that Rauber's layer either 

 takes no part at all, or only a subordinate part in the formation of the ectoderm of the 

 embryonic area. v. Beneden's view, in spite of the superficial resemblance of the ovum at 

 this stage to certain gastrula forms, has not met with general acceptance from embryologists. 



Inversion of the blastodermic layers in some mammals. In the guinea-pig 

 (Bischoff), rat and mouse (Eraser, Selenka), and in some other rodents, an inversion of the 

 usual position of the blastodermic layers is found to occur, the epiblast being innermost, 

 the hypoblast outermost. The foundation of this inversion is laid early by a process of 

 invagination and formation of a central cavity in the mass of entomeres, so that when the 

 blastoderm is differentiated, the innermost cells which are next the (secondary) cavity thus 

 formed become the epiblast, and the outermost the hypoblast, the mesoblast subsequently 

 forming between the two by proliferation of epiblast at the primitive groove, as in other 

 mammals. (For details as to this process of invagination, the student is referred to the papers 

 by Selenka.) 



Historical. The existence of several laminae in the germinal substance of the egg was 

 first suggested by C. F. Wolff in his celebrated work Theoria Generations, published in 1759, 

 and in his later Memoir On the Development of the Intestine, first published in Nov. Comment. 

 Acad. Petropol. in 1767 and republished in German by J. F. Meckel in 1812. It is, however, to 

 the researches of Pander, conducted under the direction of Dollinger of Wiirzburg, and pub- 

 lished in 1817, and those of v. Baer (1826-1 837), that we owe the first consistent attempt to 

 connect the development of the several organs and systems of the embryo with the different 

 constituent parts or layers of the blastoderm. Pander recognised a trilaminar structure of the 

 blastoderm and distinguished the three layers composing it, in their order from above down- 

 wards, or from without inwards in the egg, as the serous, vascular, and mucous layers. 



In 1850-54 a further important advance was made in the knowledge of the constitution of 

 the blastodermic layers, by the discovery by Remak that the greater part of the middle layer 

 noon after its formation corces to be divided into two lamina^ separated by a space which 

 corresponds to the perivisceral cavity (cailom} a fact which had been partially stated by von 

 Baer. So marked a division of the middle layer and distinction of the parts which are 

 afterwards developed from its two laminae, has seemed sufficient to some authors to warrant 

 the recognition of four distinct layers in the blastoderm ; but it will be found on the whole 

 more convenient to consider the fundamental layers as only three, to which, following the 

 nomenclature of Foster and Balfour, the designations of epiblast, mesoblast, and hypoblast are 

 applied, terms which are synonymous with those of ectoderm, mesoderm, and entoderm, 

 employed by many authors. 



The terms ectoderm and entoderm were first applied to the two fundamental layers, shown 

 by Huxley in 1849 to constitute the whole body of ccelenterates, and which were correctly 

 regarded by him as homologous with the two layers of the bilaminar blastoderm, to which we 



