Embryology of the Mites. 221 



case when clusters of the ova are removed for the purpose of 

 preservation. The deposition of the ova is a somewhat 

 lengthy process, and continues almost to the time when the 

 young emerge from the first batches ; one and a half months, 

 however, are requisite for development (in the northern 

 Caucasus). Towards the end of the period of oviposition the 

 tick is surrounded by a mass of eggs, which is twice or even 

 three times the size of its body. 



This at first sight apparently paradoxical phenomenon is 

 due to the fact that the ova are heaped together in a loose and 

 not in a compact mass. 



1. Segmentation of the Ovum*. — The segmentation of the 

 ovum of Ixodes differs from what was previously observed in 

 the case of the Mites, in that the cells in process of division 

 do not at first pass to the surface of the ovum (as is the case 

 in Tetranyclius^ according to Clapar^de), though the yolk 

 does not divide at the same time (as was observed by Robin 

 and Megnin in Sarcoptidge) , since I found in the yolk four 

 and six cells in course of fission, and on one occasion nine 

 with resting nuclei. The process of segmentation conse- 

 quently belongs to the partially interlecithal type. In the 

 end the cells pass to the surface and form the blastoderm ; 

 no cells remain behind in the yolk. 



2. Formation of the Endoderm. — Although in the first stages 

 of development differences are noticeable between the indi- 

 vidual cells of the blastoderm, they are at all events not 

 pronounced and are connected by transitions in such a way 

 that it is impossible to describe them precisely : the con- 

 ditions, however, soon change. Certain cells are distinguished 

 by their large nuclei, which usually occupy an oblique posi- 

 tion with reference to the surface of the ovum ; these nuclei 

 take a fainter stain from carmine than those of the ordinary 

 blastoderm cells, and contain in their interior from one to two 

 nucleoli, which stand out sharply and are not present in the 

 nuclei of the rest of the cells of the blastoderm. In one and 

 the same series of sections, but still better in series which 

 follow one another in succession, we observe how cells of this 

 kind recede from the surface into the interior of the yolk 

 and how the surrounding cells of the blastoderm close up above 

 them. These cells constitute the so-called yolk-cells. As 



* Before the segmentation I also observed the extrusion of two direc- 

 tive corpuscles ; in Ixodes this takes place prior to the deposition of the 

 ova. 



