22 Dr. W. Saleusky on IJtickeVs Gastitca Theory. 



8CSS0S no intostiiK') is ovorloa|K'cl. Tlic production of the 

 Giu^tniUi from the Jihisfula-Wmn may be regarded as an 

 abritli^ment of develoi)ment. 



Hitherto we have considered tiie processes of differentiation 

 of the gcrm-himelhv>, and seen that these may be referred to 

 two forms — the Planula and DibUistida. From this point 

 further jirocesses occur in different manners in different 

 animals, the consideration of whii-h may assist us in obtaining 

 a n*»tion of the import of tiie 6'f?.s7/-«/«-stage in the ontogeny 

 of animals. 



Let us commence om* examination with the animals which 

 in their development j)a8S through a Planula-stiv^c in the pure 

 form — that is to say, ni the early period of their development 

 j)resent a body whicli consists of two or three germ-lamellaj 

 and has no cavity in its interior. It is in this form that the 

 embryos of the Cadentcrata quit their egg-envelopes ; and in 

 this stage they have long been known luider the above name. 



The development of the GastniJa from the Plannhi has been 

 most accurately investigated by Iliickel in the Sponges, and 

 described in detail in his Monograph. The phenomena of 

 this process are as follows : — First of all a cavity is formed in 

 the entoderm of the Phimda ; this stage Iliickel indicates by 

 the name of " PlanojjantruJay The mouth then breaks into 

 this cavity from without, by which the conversion of the 

 Planogastrula into the Gastrula is effected. In the hydroid 

 polyjics this conversion has long been known. By the con- 

 version of the Plamda into the Gastrula the chief processes of 

 the fonnation of the body of tlie Coclcnterate are already 

 completed. The body in these animals (llydroid Polypes, 

 Sponges, &c.) consists during their whole life of these two 

 layers which circumscribe tlie cavity ; only the organs which 

 distinguish the different groups of the Coclenterata from each 

 other (tentacles, pores, skeletal parts, &c.) are afterwards 

 formed. 



It is probable that in the Turbellaria also similar simple 

 processes occur in the formation of the stomachal cavity ; but 

 as very little is known about tlieir development, we cannot 

 affirm this. According to Kefersteiji's statements, the 

 stomachal wall of these animals is produced by the differentia- 

 tion of the superior layer of cells (exodcrm). Unfortunately 

 we do not know the developmental history of the aeoeloua 

 Turbellaria {Convolufa, Srhizo/)rora, &c.). In their organiza- 

 tion these are distinguished from the Planula only by tlie 

 presence of the nioutli. From this it may be supposed that 

 tiie conversion of these animals from t\ic Planula consists only 

 in the perforation of the buccal oriticc. 



