220 NIKOLAS KLEINENBERG. 



internal development, I shall take no more notice of it and 

 I shall treat of each embryo without heeding its companion. 

 We left the embryo in the form of a depressed globe, now it 

 is lengthened in its antero-posterior diameter, and a little 

 compressed on the dorsal and ventral surfaces, and hence has 

 the shape of an oval lens. The central cavity enlarges be- 

 cause it begins to suck m to itself part of the albumen in 

 which the embryo swims. This nutritive substance does not 

 become employed and transformed immediately into the 

 growing tissues, but, drawing itself together, forms a large 

 and dense mass, which nearly completely fills the space. 

 The mouth, although it serves as a passage for the introduc- 

 tion of the albumen, becomes diminished to a very fine canal, 

 which pierces the body-wall obliquely from below upwards. 

 Sometimes it shuts completely, and then, being without the 

 means of absorbing the albumen, the embryo remains very 

 small and the lumen of the canal disappears, its walls ap- 

 proaching each other till they touch. Notwithstanding this, 

 all the tissues develop regularly and arrive at perfection, if 

 in the subsequent changes the mouth reopens. 



The Germinal Layers and the Germinal Streaks. 



The way in which the blastomeres of one hemisphere be- 

 come arranged in distinct layers, while the common rudiment 

 of the two embryos is still a solid sphere, has been described 

 above. The ectoderm {ec in all the figures) becomes defined 

 by the separation of a single layer of cells around a solid 

 central mass. Its cells from the first are cylindrical, with 

 rather dense protoplasm, containing a great number of very 

 fine granules. As the embryo increases in size the cells 

 multiply and, losing their cylindrical form, become trans- 

 formed into very broad and thin plates, which cover, as a 

 single layer, the whole body of the embryo. In the middle 

 line of the ventral surface a double or treble row of these 

 cells, stretching from the aboral pole to the mouth, developes 

 a great number of vibratile cilia, which produce by their 

 movements the continual gentle rotation of the embryo about 

 its transverse axis. 



The formation of the endoderm {en) is not so simple and 

 easily explained. It appears possible, even probable, that 

 when the germinal bladder (fig. 2) becomes solid some of 

 the lower and smaller cells of one pole enter into the 

 segmentation cavity ; but, on the other hand, there is no 

 doubt that other cells, which participate in the formation of 

 the inner layer, separate themselves from the central ends of 

 the long cells surrounding one side of the segmentation 



