DEVELOI'MKNT IN PLUM ULARTA. C9 



becomes somewhat I'-shaped. The ovum has now entered the 

 ectoderm, and the covering cap becomes divisible into an inner 

 columnar la^'er in immediate contact with the ovum, and an 

 outer layer of flat cells. The ectoderm at the sides of the 

 .^onotheca .s^radually contracts away from the chitinous layers. 

 The diameter of the ovum is now about 20 /i. 



The gonotheca continues to expand and the egg grow> 

 sliglitly (diameter about 23/A) ; no obvious yolk is passed into it. 

 The covering cap of ectoderm cells of two layers, which pro- 

 bablv represents a rudimentarv gonophore, also grows. A 

 delicate mesoglea layer appears in this region, and the endoderm 

 cells on which the ovum is seated are granular and stain more 

 leadilv than the remainder of the endoderm. 



The gonotheca expands further, and the distal plate of tall 

 columnar cells and granular cells begins to secrete a thicker 

 layer of perisarc ; this is the beginning of the operculum. The 

 blastostvle. except in the region of the opercuhnn, is quite 

 .'-eparated from the chitinous layer of the gonotheca and lies if- 

 the mid-axis. The cap of two layers of ectoderm around the 

 ovum grows and becomes separated from the ovum, so that there 

 is formed a distinct space above the egg. 



The egg is probably fertilized at tliis stage, and iM-esumably 

 after such fertilization it secretes a kind of vitelline membrane, 

 which is thicker on the outer side tban on the inner side. The 

 endoderm becomes slightly pushed out into a blunt process con- 

 sisting of narrow granular cells, and the egg is seated on this 

 out-pushing. The ovum reaches a diameter of about 34/^. 



The gonotheca now grows to its full size, and the cells of 

 the covering plate and of the endoderm. become attenuated and 

 thin. The space between the egg and tlie ectoderm layers repre- 

 senting the gonophore increases in size. The vitelline membrane 

 becomes thicker. The operculum consists of thicker perisarc 

 than the rest of the gonotheca, and its edge is sharplv marked 

 off from the surromiding thinner portion below. The gonotheca 

 has now assumed its definite form and size and further growth 

 does nor occur; its greatest width is about 0.48 mm. and length 

 o.Sg mm. 



At a somewhat later stage the outermost gonoijhore layer 

 of the cells disappears, and the inner la3^er of cubical or columnar 

 cells breaks up to form an irregular cluster around the 

 ovum. The covering plate of ectoderm cells below the oper- 

 culum and the endoderm layer become still more attenuated, and 

 will shortlv disappear. The outpushing of endoderm becomes 

 rather more pronounced, and the cells remain granular and 

 actively living. The egg does not grow, the protoplasm is finely 

 granular, and there is a large nucleus with nucleolus. There is 

 still a pronounced membrane around the egg : it is less obvious 

 in the inner side against the granular cells of the endodermal 

 outpushing. There is no obvious mesoglea separating the egg 

 from the endoderm. 



