588 



HAROLD HEATH, 



First Quartette of 

 Ectomeres 



Second Quartette of Third Quartette of ., ,, , 



., ^ ,, ^ Mesomcre Entomeres 



Eitnmeres Üctomeres 



1. Secondary trocho- 

 blasts. 



2. First somatoblast. 



3. Portions of shell and 

 foot. 



4. General ectoblast 

 lateral to mouth. 



5. Stomatoblasts. 



6. Post-trochal cells 

 lorming the "sup-, 

 porting layer" of 

 the velum. 



1. Stomatublasts. 



2. Lateral regions ot 

 the shell, probably. 



3. Lateral regions of 

 ' the fool 



4. Larger portion of 

 general trunk ecto- 

 blast (diagram E). 



Mesoblast 

 bands. 



Meseuteron 



1 Primary and acces- 

 sory trochoblasts. 



2. Apical sense organ. 



3. Cerebral ganglia. 



4. Molluscan cross and 

 rosette series. 



5. General ectoblast of 

 upper hemisphere. 



6. Tlie proboscis of the 

 adult. 



7. Major portion of tlie 

 first valve of shell. 



8. Mantle and mantle 

 furrow in region of 

 6 and 7. 



CoNKLiN ('97) has shown in Crepidula that some of the products 

 of the first quartette (part of the posterior arm of the cross) pass 

 down into the posterior hemisphere. Mead finds the same thing to 

 be true in Amphitrite, though probably to a less extent. In IscJino- 

 cJiiton on the other hand cells of the first quartette becoming ciliated 

 aid in filling up the posterior gap in the prototroch, but none pass 

 below it. Hence in a consideration of this quartette we deal with 

 the anterior hemisphere whose lower boundary is the posterior border 

 of the prototroch. 



For purposes of convenience the tip cells of the cross, since they 

 enter into the formation of the velum, will be considered in this con- 

 nection though it should be borne in mind they are second quartette 

 cells. 



I. Trochoblasts and TipCells; Formation of Prototroch. 

 To recapitulate briefly, the first division of the first quartette 

 gives rise leiotropically to the primary trochoblasts (Fig. 7). These 

 are formed at the same time as the second quartette and with these 

 latter cells alternate about the equator of the embryo. The next 

 division affecting these blastomeres occurs in the 32 cell stage when 

 each cell cleaves dexiotropically into two products of equal size. Al- 

 most immediately after this cleavage each of the four cells of the 

 first quartette, corresponding to the intermediate girdle cells of An- 

 nelids {lai-'-^ etc.), divides in a leiotropic direction, giving rise to a 

 cell which lies to the left of the two primary trochoblasts of each 

 quadrant (Fig. 15). Of about the same size as either primary trocho- 

 blast it also subsequently undergoes similar changes and finally enters 



