EAKLY DEVELOPMENT OF CRIBRELLA OCULATA. 381 



produce the invaginated heads in cystic stages of Taenia. In each case an organ which 

 belongs normally to the outer surface is first produced in the interior, and is afterwards 

 rapidly translated to its normal position by egression. 



Immediately following upon the process of egression is that of invagination. The 

 final result of complete egression is the production of a monoblastic embryo corresponding 

 to a blastula, but this stage, if attained at all, is very rare. All gastrulse which I have 

 examined show, as in fig. 6, the egression-grooves radiating outwards over the surface 

 of the embryo. The blastopore is at first very large and wide, but soon narrows con- 

 siderably to the condition found in fig. 6. From this stage onwards its edges gradually 

 approximate, until it is reduced to a small circular hole at the hind-end of the embryo. 

 In many cases it persists in this condition through stage A of the larva, but usually it 

 closes at the hatching period. 



A section through the gastrula (fig. 18) shows the epiblast in places still exhibiting 

 evidence of the process of egression. Between it and the hypoblast is a definite archi- 

 ccele cavity, which is practically free of cells. The hypoblast itself is many-layered. 

 Cell-production is evidently proceeding rapidly, in earlier stages mainly on the inner 

 surface of the hypoblast, but later upon the outer. The cells produced from the former 

 fall free into the archiccele cavity, and give rise to a protoplasmic network similar to the 

 mesenchyme of other larvae (Antedon), whilst those produced upon the latter fill up 

 the archenteric cavity with a network of protoplasm which is histologically identical 

 with the true mesenchyme. There does not appear to be any special centre for the 

 production of mesenchyme, nor anything in the nature of pole-cells. For the mass of 

 protoplasm and nuclei, corresponding in origin and in histological structure with the 

 mesenchyme, but filling the archenteric cavity, the name of ' hypenchyme ' is suggested. 



We may now examine in greater detail the last embryonic stage, as shown in fig. 19. 

 The embryo is spherical, or nearly so, as in many cases the anterior end can be recognised 

 by a very slight attenuation. At the posterior end is seen a small aperture, the 

 blastopore. A median section is seen in fig. 26. The epiblast is now completely 

 consolidated, and there is no trace of egression; each epiblast cell (fig. 27) is of great 

 length, and scarcely wider than its own nucleus. The nuclei are aggregated in a zone 

 about half-way along the cell, whilst scattered throughout the cytoplasm are numerous 

 yolk-globules. These globules are small and few in number outside the nucleus, but are 

 large and numerous in the inner parts of the cells. This arrangement results in the 

 epiblast having a darker appearance in its peripheral part when examined under the 

 low power. The cell-walls are perfectly definite, though very delicate ; and the same 

 remark applies to the basal wall dividing the epiblast from the mesenchyme. Externally 

 the epiblast is covered by a thick cuticle of a pale yellow T refractive appearance. At the 

 blastopore the epiblast is merged into the hypoblast, and the cuticle is disintegrated into 

 small fragments, which lie loose in the opening. 



The hypoblast is now in the shape of a club with a large head and a constricted 

 stalk. The structure is the same throughout, and is shown in fig. 28. Each cell is 

 TRANS. ROY. SOC. EDIN., VOL. XL. PART II. (NO. 19). 3 m 



