106 INVERTEBRATA CHAP. 



whilst B is median and anterior. B and D (in Planocera) are larger 

 than A and C and meet eacli other in short contact plane at the 

 vegetative pole, whilst A and C meet each other in a short contact 

 plane at right angles to this at the upper or animal pole of the egg. 



Now the division of these four cells used to he described so that A 

 was said to hud off a micromere a ; B a micromere b ; and so on. 

 But this is not a logical description of the event, for the so-called 

 budding off of a is really the division of A into two cells, a larger 

 and a smaller, and neither should hear the same name as belonged 

 to their common mother. So that under an improved nomenclature, 

 when the four first blastomeres divide, the cells to which they give 

 rise are denominated la and 1A, ll and IB, Ic and 1C, and Id and 

 ID respectively ; the smaller letter in each case denoting the 

 micromere and the larger the macromere (Fig. 80). 



When the second group of micromeres is given off, 1A divides 

 into 2 and 2A, and so on, but to denote the divisions of the already 

 formed micromeres a different notation is used. Thus la is said to 

 divide into la 1 and la 2 , where l denotes the daughter nearest the 

 animal pole, and 2 the one nearest the vegetative pole of the egg 

 (Fig. 81). When in the 16 -cell stage la 1 divides, its daughters 

 are denoted la 11 and la 12 , on the same principles. 



It will easily be seen that this notation is capable of indefinite 

 expansion. It has, however, one serious defect. Sometimes the two 

 daughters resulting from the division of a cell lie side by side 

 at the same distance from both poles of the egg. In this case 

 1 is held to denote the right-hand daughter, and 2 the left- 

 hand daughter. But this is confusing, because the eye learns to 

 associate the symbol ' 2 with a lower position in the egg than 1 , 

 and it is a strain to grasp the fact that the cell 2a- u may be higher 

 in the egg than 2 m . 



Woltereck in his paper on Polygordius (see Chapter VII.) 

 gets over this difficulty by using the letters (r} and (/) to denote 

 right and left instead of the numbers l and 2 , if the two daughters 

 lie to the right and left of the median plane of the egg. Where, 

 however, they both lie at the side of the egg, then the letters (a) 

 and (p) for anterior and posterior are employed. This practice 

 completes the perfection of the nomenclature, but it is unfortunately 

 not employed by Surface and the other American writers. 



Each group of micromeres is known as a quartette, and all the 

 cells resulting from the divisions of one of the first four blastomeres 

 are known collectively as a quadrant of the egg. 



To return to the special case of Planocera. The cell D is distinguish- 

 able as the largest of the first four, and, as we have seen, it occupies 

 what afterwards turns out to be the posterior pole of the embryo. 

 When once this is recognized the egg is so placed that D is posterior. 



In the formation of the first micromeres D divides first, B 

 follows, and A and C divide simultaneously, so that division takes 

 place in the order of the size of the cell. The first micromeres 



