Miscellaneous. 191 



Tho propfony of the protoplasmic amoeboid cell multiply by divi- 

 siou, and travel from the ccutre of the ovum to its periphery; only 

 very few of them remain in the interior, surround the large excen- 

 trie fat-globule, and disintegrate the fat- and albumen-globules. A 

 small number only of tho cells which travel out to the periphery of 

 the ovum actually emerge at a few points on its surface ; many of 

 them merely approacli the periphery and then undergo active multi- 

 ])lication, whereby the limits of certain cells become confluent and 

 thus form " plasmodia," in which the nuclei are accumulated in 

 heaps and form groups or nests. 



Both the cells which have gained the surface of the ovum, as well 

 aa those forming plasmodia, undergo further division, and, as the 

 former extend over the surface of the ovum, while the latter emerge 

 thereon, constitute a continuous b/astodermal layer. The cells of the 

 blastoderm are everywhere similar in size and form, and the ovum 

 haa now reached the hUistida stage. On further development the 

 cells of one half of the ovum become columnar, wiiile those of 

 the other half retain a cuboid shape. The columnar cells form an 

 elongated streak — the primitive streaJi.', and the ovum enters upon 

 the stage of the jjolar hlastula. The embryo now exhibits bilateral 

 symmetry ; we can trace the dorso-ventral plane, and distinguish 

 the ventral and dorsal sides of the embryo, but the anterior and 

 posterior ends of it are as yet alike undistinguishable. At a furtlier 

 advanced stage the flattish cells in the median line near one end of 

 the dorsal side of the embryo become columnar, and we get a local 

 blastodermal thickening, which constitutes an apical plate ; with the 

 appearance of this latter organ we are in a position to distinguish 

 tho anterior end of the embryo, and to fix the position of all the 

 other regions. In this com_pletehj bilateral stage the embryo re- 

 tains, as before, a perfectly spherical shape, but its diameter is now 

 greater. 



The germinal layers arc formed by invagination. The blastopore 

 is a slight hollow, below which there lie a very few amoeboid cells, 

 which slowly sink into the yolk. Although I have made a number 

 of preparations of this stage, I have never succeeded in discovering 

 merely a hollow before the separation of the cells, but always found 

 the hollow, with the cells lying beneath it. I never once detected 

 the process of mitosis in the cells of the hollow; this is very diffi- 

 cult to observe on account of the small size of the nuclei. 



The cells which underlie the hollow together constitute the meso- 

 endoderm, which soon differentiates into two separate and indepen- 

 dent germinal layers. The endoderm takes the shape of a solid 

 cord, in which the cavity subsequently appears, and the two ends of 

 which abut upon the rectum and the oesophagus. The rectum, as 

 well as the oesophagus, arises as an invagination of the ectoderm : 

 the former lies behind the blastopore, the latter in front of it, oppo- 

 site the apical plate. The whole of the endoderm-cells do not take 

 part in the formation of the mesenteron ; some of them spread over 

 the nutritive yolk, and form two large symmetrically placed pro- 

 visional hepatic sacs. 



