Germinal Streak of Mysis. 189 



not keep pace with that of the ectoderm, so that the divisions 

 of tlie germinal streak wliich contain proto-scgments alter- 

 nate with others in which no elements of the muscle-plates 

 are to be found: by this means this " primitive segmentation" 

 can be recognized with peculiar distinctness. Each proto- 

 scgment at first consists of a simple transverse row of cells : 

 it is not until later on that these multiply, so that the rudi- 

 nient gradually comes to consist of several rows and several 

 layers, when, owing to the fact that the proto-segments then 

 fuse together, the muscle-plates develop into a continuous 

 layer within the ectoderm. 



The ingrowth alluded to above, through which the deeper 

 cell-layers of the embryo are formed, without doubt corre- 

 sponds to the gastrula-invagination, from the lateral margins 

 of which the Ibrmation of the muscle-plates consequently 

 proceeds in this case also. Whether these muscle-plates 

 belong genetically to the ectoderm or to the endoderm it was 

 impossible to decide, and the question is one of those which 

 in many cases are most difficult of all to determine, but never- 

 theless are often " decided " with the utmost arbitrariness. 

 The blastopore has no relation whatever either to the mouth 

 or anus; its situation is in the neighbourhood of the future 

 anus : this, however, does not arise until much later, long 

 after the blastopore has become completely unrecognizable. 

 Before the formation of the gastrula-ingrowth no yolk-cells 

 are to be found. 



At the anterior margin of the blastopore a very peculiar 

 differentiation of certain ectoderm-cells takes place : these 

 develop into primitive cells of the ectodermal portion of the 

 germinal streak. The definitive number of these cells is 

 seventeen or nineteen (I find sometimes the one, sometimes 

 the other) ; they form a transverse arcuate streak in front of 

 the blastopore. The first stages of the differentiation and 

 grouping of these cells seem tu last only a very short time ; 

 for, in sj)ite of the fact that I examined a very large number 

 of germinal disks at such stages, I am only able to assert that 

 I have found stages with nine, eleven, thirteen, and fifteen 

 primitive cells ; less than nine primitive cells were not found, 

 and consequently no transition between this stage and the 

 earlier phase, at which sucli a grouping of the cells is alto- 

 getlier indistinguishable; it was likewise impo.ssible to deter- 

 mine whether the original nine cells multiply into the seven- 

 teen or nineteen by means of fission or whether their number 

 is augmented by accessions from other neighbouring celb. 



