Royal Microscojncal Society. 197 



The elements of one and the same blastoderm exhibit a great 

 difference in size. There are always a great many elements to be 

 found apparently undergoing division (see Fig. 8 A). 



The elements of a blastoderm of a later period (for instance ten 

 days) appear somewhat peculiar in this respect. 



If the observation is continued, we can see that some elements 

 which have been at first of a spherical shape, become of a cylin- 

 drical shape ; further, we can see that on the latter there appear 

 transverse grooves at nearly equal distances, as if it would divide 

 itself into a certain number of smaller spherical elements (see 

 Fig. 8). 



About the spreading out of the blastoderm on the saucer-like 

 depression of the yolk ; about the large elements which are to be 

 Ibund on the bottom of the segmentation cavity ; about the growth 

 of the blastoderm round the yolk globe. like a cap, the peripheral 

 part of which is thickened ; about the foimdation of the embryo at 

 the peripheral thickening, I have only few things to add to what 

 has been shown by Strieker. 



Between the tenth and twelfth day the blastoderm, which in the 

 last days of the segmentation has become much larger and thicker, is 

 seen to grow very rapidly in breadth. A fresh egg shows the blasto- 

 derm in this stage more transparent at the centre than at the periphery. 

 On a section we recognize that the central portion of the blastoderm 

 is much thinner than the peripheral portion ; the central portion 

 does not touch the yolk, but is separated from the saucer-like 

 depression of the yolk by a cleft. The elements, of which the 

 blastoderm consists, are in the deeper layers of the central part as 

 well as the peripheral part larger than in the superficial ones. The 

 most superficial layer consists of very much flattened cells ; on sections 

 it appears, as if the blastoderm were covered by a thin layer of spindle- 

 shaped cells. We recognize at the same time that the blastoderm is 

 not raised from the saucer-like depression of the yolk for the whole 

 length of the cleft, that is, the segmentation cavity, but rests on the 

 latter from place to place, by means of columns of cells, as if by 

 pillars. The cells of these columns — subgerminal processes — are 

 larger than those of the blastoderm, and are coarsely granular : we find 

 cells similar to these on the bottom of the segmentation cavity too ; 

 amongst these we recognize some, which are very much larger, and 

 possess two, three, or more nuclei. There is nothing easier than 

 to convince ourselves, with Kinek,* first, that these elements which 

 lie on the bottom of the cavity are products of the blastoderm 

 itself, that is, that just as the blastoderm is lifted from the yolk, 

 there remain some elements on its saucer-like depression ; secondly, 

 * Max SchuUze's ' Archiv,' Bd. V. 



