EARLY DEVELOPMENT OF GBANTIA COMPKESSA. 275 



16. The Problem of the Polarity of the Oocyte, the Ovum, and the Embryo. 



This important problem is very difficult to understand properly, because 

 the granules in the Grantia ego- are evenly disposed, and one cannot identify 

 a vegetative or animal pole by this means. In fact, one is obliged to 

 consider that the egg of Grantia is a primitive amoeboid body, without 

 visible signs of a polarity such as one finds in the amphibian egg (PI. 19. 

 fig. 9). 



In most cases the egg of Grantia lies beneath the gastral epithelium in 

 such a way as to be compiressed into an oval form, as shown in PI. 19. fig. 9 ; 

 the nucleus itself is rarely quite spherical, being oval in the same direction 

 as the egg. While some oocytes at this stage are quite spherical, and their 

 nuclei likewise, it is true that by far the majority of Grantia oocytes possess 

 this form of bilateral polarity. While the oocyte nucleus is most commonly 

 oval or elliptical, the pronuclei are often quite spherical. Though it is 

 difficult to come to a conclusion, I fear that the above observations on the 

 gross polarity of the oocyte are of only small value. 



The ectoplasm, too, lies around the entire periphery of the egg, and lends 

 no clue to the solving of this problem. Polar bodies are most often given 

 off towards the gastral side of the egg, but I know of one case where a 

 polar body was passed out on that side of the egg lying towards the mesogiea. 

 I do not consider that much significance can be attached to this. 



I have been forced to the conclusion that, apart from the bilateral polarity 

 caused by pressure, the oocyte .shows no permanent signs of definite polarity. 



Now during fertilization the sperm always, in my experience, enters at 

 that side of the egg nearest the gastral cavity (PI. 19. fig. [), S), and in by 

 far the greater number of examples the penetration of the sperm causes a 

 partial flow of cytoplasmic granules to that side of the oocyte on which 

 the sperm entered (PI. 20. fig. 13). The egg thus comes to have a definite 

 polarity temporarily conferred by the entry of the sperm. The question 

 then arises as to whether the place of entry of the spermatozoon marks 

 the animal pole, and whether it is the animal pole which later becomes the 

 flagellated hemisphere of the blastula ? 



There are several established (acts which we may notice at once : The 

 flagellated half of the amphiblastula larva almost always becomes formed 

 from that hemisphere of the undifferentiated blastula abutting against the 

 gastral layer ; the sperm always enters on this same side, and the polar 

 bodies are most often given off on this side ; in the differentiating blastula 

 (PI. 22. fig. 27), the line of demarcation between future flagellate cells and 

 future granular cells is sharply distinguishable. 



I may say, nevertheless, that I am aware that the above evidence is 

 insufficient fully to settle the question of the polarity. One may, however, 

 follow out this line of reasoning to its fullest extent, though it should be 



