10 e. 0. Whitman 



obtusely rounded, generally broader thau long, and 

 but little wider thantbebody. Tbe propolar cells differ, 

 as a mie, but little in size from the metapolar cells. The 

 diameter of the propolar set varies but little from that 

 of the metapolar set, except in the plagiotropal calotte. 

 No marked and Constant difference in size between the 

 ventral and dorsal polar cells. Parapolar collar ad- 

 joiued by 2 dorsal and 1 ventral ectodermal celi. 



Verruciform cells — 5. 



Total number of ectodermal cells 25. 



The description of this species given by Van Beneden , is in the 

 main correct, but, I must add, it seems to bave been drawn from a study 

 of few specimens , and certain points bave been strongly ernjibasized 

 which bave no specific vaine , while others appear to bave been misap- 

 prehended. I bave seen calottes similar to the one so accurately and 

 lucidly described by Van Beneden ; but this calotte does not , in many 

 respects, represent characters that can be said to hold true of the 

 species. In this calotte , tbe ventral cells are smaller than the corre- 

 sponding dorsal ones , and we are told that this feature characterizes 

 the species. Hence it is made the starting-point for general orientation, 

 and for explaining the obliquity and bilateral symmetry of the head. 



My observations bave convinced me that the size of the polar cells 

 is a very unsafe guide in determining the dorsal and ventral sides of 

 the calotte. In the orthotropal calotte, the only reliable means of orien- 

 tation which I bave been able to discover lies in the relative position of 

 the parapolar and the three proximate ectodermal cells. In the plagiot- 

 ropal calotte, it is not the size of the cells, but their oblique arrange- 

 ment that enables one to recognize at a glance the different sides of the 

 head. A comparison of the figures given of this species will show that 

 the obliquity of the head is by no means to be attributed to a difference 

 in size between the dorsal and ventral cells ; and further, that it will 

 not do to say that the propolar cells are invariably smaller than the 

 metapolar. 



Fig. 17 represents an unliberated embryo, .25 mm in length, seen 

 from the ventral side, as shown by the arrangement of the three cells 

 immediately foUowing the parapolar cells. The inclination of the head 

 and the loosening of the cells are due to the action of acetic acid. 



Fig. 16 represents an osmic acid preparation of ayoungRhombogen 

 /.6 mm), from the left side; and Fig. 19 (1 mm) another from the same 

 side. There is no noticeable obliquity of the calotte in either of these 



