FORMATION AND FATE OF THE PEIMTTIVE STREAK. 453 



None of the ova were stained, for our experience has been 

 that the solutions through which the ova are passed, before the 

 staining is complete, alters the relations of the cells, and that 

 the staining hinders rather than facilitates the microscopical 

 examination of sections of the younger stages. 



We obtained several surface views which were useful for com- 

 parison with the results of previous observers ; of these the one 

 represented in fig. 11 was drawn from a living embryo. 



Fig. 23 was drawn partly from a surface view of an ovum 

 hardened in Kleinenberg's fluid, and partly from a model con- 

 structed by putting together, in order of sequence, pieces of 

 cardboard cut to correspond with camera drawings of a com- 

 plete series of transverse sections. Our work was greatly 

 facilitated by Professor Marshall, who very kindly placed at 

 our disposal a large series of sections, and to him also our 

 thanks are due for much kind advice. 



Before proceeding to the description of our own observations, 

 we must refer shortly to previous records, for by this course 

 alone shall we be able to indicate clearly the differences be- 

 tween our results and those of the observers who have pre- 

 ceded us, and at the same time we shall obtain an opportunity 

 of defining some of the terms that we shall be obliged to use in 

 our description. 



Turning, therefore, in the first place to the consideration of 

 the formation of the archenteron and the blastopore (but 

 leaving aside for the present the concrescence theory of His 

 [22 and 23], so far as it concerns the formation of this cavity), 

 we find that upon this, as upon most other important points, 

 there is a distinct difference of opinion between the previous 

 observers. It is stated by some that after the formation of the 

 segmentation cavity, and when the ovum is only partially 

 covered by the pigmented epiblast-cells, the archenteron is 

 formed by an invagination, which "first commences by an inflec- 

 tion of the epiblast-cells for a small arc on the equatorial line 

 which marks the junction between the epiblastic cells and the 

 yolk-cells^^ (1, p. 102), 



This preliminary invagination is also described by Perenyi 



