JOURNAL OF MICROSCOPICAL SCIENCE. 



EXPLANATION OF PLATE XXV, 



Illustrating Professor Ray Lankester^s Memoir " On the 

 Coincidence of the Blastopore and Anus in Paludina 

 vivipara/^ 



Pig. 1. — Uusegmeuted egg as found iu tlie upper part of the oviduct. 

 The germinal vesicle is not observable : the protoplasna is charged with 

 food-material in the form of yellow granules. As seen in the living state 

 by transmitted light. Hartnack's Oc. iv, Obj. 8. 



Fig. '2. — Early stage of cleavage, shovping the hyaline character of the 

 protoplasm at the edges of the large cleavage-spheres and the contained 

 yellow food -granules. As seen iu tiie living state by transmitted light. 

 Hartnack's Oc. iv, Obj. 8. 



Fig. 3. — Blastula stage. The cells have now further divided, and the 

 embryo assumes the form of a hollow sphere, the wall of which consists of 

 a single layer of cells. As seen in the living state by transmitted light. 

 Hartnack's Oc. iv, Obj. 8. 



Fig. 4. — Commencement of the gastrula-iuvagination. Bl. Blastopore. 

 Dc. Directive corpuscle (this is omitted in the previous figures). As seen 

 in the liviug state, with slight exaggeration of the transparency. Hart- 

 nack's Oc. iv, Obj. 8. Ed. Ectoderm or epiblast. End. Endoderm or 

 hypoblast. Bl. Blastopore. Be. Cavity of the blastula- or segmentation- 

 cavity. 



Fig. 5, — Completion of the gastrula. The endoderm, e«6?, has now applied 

 itself closely to the ectoderm, and bounds the primitive stomach or 

 archenteron. Bl. Blastopore. Bhn. Margin of the blastopore. Ed. Ec- 

 toderm. End. Endoderm. Dc. Directive corpuscle. As seen in the living 

 state by transmitted light. Hartnack's Oc. iv, Obj. 8. 



Fig. 6. — Diagram of median vertical section of the specimen drawn in 

 Fig. 5. Bl. Blastopore. Ae. Archenteron. Dc. Directive corpuscle. 



Fig. 7. — Trochosj)here stage. A circlet of cilia now divides the embryo 

 into unequal anterior and posterior regions. A space, the mesoblastic 

 space or coelom traversed by delicate filaments connecting endoderm and 

 ectoderm, is now apparent. The blasto])ore is still widely open. Dv. Dome 

 of the velum area. Fr. Ridge or ciliated margin of the velum area. 



This specimen is viewed from the pedal or ventral surface ; that on which 

 both mouth and anus subsequently lie. Seen by transmitted light with 

 Hartnack's Oc. iv, Obj. 8. 



Fig. 8. — Median vertical dorso-ventral section of a similar embryo as 

 seen in a living specimen, the transparency of the parts being slightly 

 exaggerated. Do. Dome of the velum area. Vr. Ridge of the velum. 

 Bl. Blastopore. F. First indication of the foot. Sm. Position subsequently 



