10. EMBRYOLOGICAL DEVELOPMENT 105 



homogeneous and semi-transparent; the opposite end stains heavily and 

 appears granular because of the mass of cellular inclusions. The cytoplasm 

 is filled with yolk granules and oil droplets, whose number and size drop off 

 sharply towards the homogeneous end of the egg (Fig. 715). 



Study of the later stages of development shows that the homogeneous clear 

 end of the egg becomes the anterior end of the animal. It has not been estab- 

 lished whether the convex or the concave side of the egg forms the dorsal 

 side of the animal. Since the egg, in addition to its anterio-posterior polarity 

 shows a distinction between a concave and a convex surface, it possesses a 

 definite bilateral symmetry, which is reinforced by the displacement of the 

 nucleus and its immediate perikaryon, poor in inclusions, to the concave 

 side (Fig. 715). 



The egg is deposited into the tube before fertilization. The nucleus im- 

 mediately migrates to the anterior end into the region which is free of 

 cytoplasmic inclusion and there apparently undergoes reduction division. 

 Fertilization then takes place in the tube, while the nucleus of the ovum is 

 at the anterior end. The fertilization membrane is then formed and the 

 fertilized nucleus returns to its former position in the middle of the egg. 

 Because of this return it was not at first realized that the polar bodies are 

 actually formed near the anterior pole, despite their being found there. 



There are thus two indications — the position of the polar bodies and the 

 distribution of food reserves — to show that the animal-vegetative axis of the 

 egg coincides with its antero-posterior axis. The main body axis of the adult 

 pogonophore thus corresponds to that of a protaxonic animal. 



Cleavage and gastrulation 



The egg undergoes total unequal cleavage. As the first sign of impending 

 cleavage it becomes noticeably shorter. The first plane of cleavage runs 

 obliquely across the egg and divides it into two equal-sized blastomeres 

 (Fig. 72). At the second division both blastomeres divide simultaneously, 

 each into two unequal cells. Of the four cells resulting, the two larger lie at 

 the anterior and posterior ends of the egg and the two smaller amidships, side 



0.1mm 



Fig. 72. Cleavage of the egg of Siboglimim caulleryi: 2-cell stage. (After Ivanov, 1957b.) 



