180 



INVEETEBEATA 



CHAP. 



Eeichenbach found that in the first stage observed by him the egg 

 was imperfectly divided by radiating planes into a series of radially 

 arranged pillars, in each of which was contained one of the daughter 

 nuclei. These pillars were referred to by previous authors as 

 " primary yolk pyramids." Eeichenbach regards them correctly as 

 an imperfect division of the egg into columnar blastomeres ; the 

 cleavage planes which separate adjacent pillars correspond to the 

 planes which divide adjacent blastomeres in other eggs. He shows, 

 indeed, that each pillar of yolk is capped on its external surface by 

 cytoplasm containing a nucleus, and is clothed also on its sides with 

 cytoplasm. 



In Eeichenbach's first stage, then, we have a blastula in which the 

 blastocoele is filled with unsegmeuted yolk. The yolky part of the 

 blastomeres, the yolk pyramids, persist as such for a very short time ■; 

 the dividing planes disappear, and we are left with a skin of flattened 



cells surrounding an immense mass 

 of yolk. Such a skin is termed a 

 " blastoderm." 



The formation of the gastrula 

 is initiated by an increase in num- 

 ber of the blastoderm cells on one 

 side of the egg. They press on each 

 other laterally and become columnar 

 in character, and so the " ventral- 

 plate " is formed. This ventral 

 plate indicates the future neural 

 side of the embryo. Strictly speak- 

 ing, all cells within the confines of 

 the plate have not the columnar 

 character; this is confined to five 

 circular areas, in each of which 

 the cells are arranged in elegant 

 concentric curves and in lines 

 radiatmg from a central point. Of these five areas the two anterior 

 and widest apart are termed the " cephalic lobes." They are the rudi- 

 ments of the paired eyes and of the cerebral ganglia, and in the centre 

 of each IS to be found a pair of cells larger and clearer than the rest. 

 Behind the cephalic lobes, and situated so close together as almost 

 to touch one another, are two similar areas, which Eeichenbach terms 

 the thoracico-abdominal rudiments ; and behind these again, in the 

 middle line, is a single circular area, the endodermic rudiment. At 

 the front border of the endodermic rudiment the cells are engaged in 

 active proliferation, and here they are not in a single layer but in 

 several layers of small rounded cells. This is the point of origin of 

 the mesoderm. 



In the next stage the areas of the ventral plate which intervene 

 between the five circular areas shrink so as to bring these latter closer 



Fig. 128.' — Sagittal section through the 

 blastula of Astacus fluviatUis to show 

 the primary yolk pyramids. (After 

 Eeichenbach.) 



Letters as in previous figure. 



together. 



This shrinkage is almost certainly due to a change in form 



