NOTES ON ESIBRYOLOGY AND CLASSIFICATION. 4l5 



delamination, and of the hypothesis of a primary invagination. 

 We shall now briefly state the difficulties which are en- 

 countered by HaeckePs Gastrsea theory in reference to the 

 earlier stages of development^ and point out to what extent 

 these difficulties are avoided by the Planula theory. 



If we assume with Haeckel that the process of invagina- 

 tion represents the historical mode of the formation of the 

 enteron, and that the blastopore is the primitive mouth, we 

 meet with difficulty as to (1) the transition from the incep- 

 tive nutrition of amoeboid cells to the absorptive nutrition of 

 the cells lining a more or less completely enclosed digestive 

 chamber; (2) the substitution, by a process of retardation, 

 of delamination in a few rare cases for the supposed more 

 archaic invagination ; (3) the disappearance, in some cases, 

 of the supposed primitive mouth, and the formation of a new 

 secondary mouth, whilst in closely allied forms the supposed 

 primitive mouth persists as mouth, or again ceases to be 

 mouth and becomes anus, whilst a new mouth develops ; the 

 result being that the mouth of one Gastropod (to take an 

 example) has to be considered as the homologue of the anus 

 of another. 



On the other hand, the hypothesis of primary delami- 

 nation and secondary invagination gives an intelligible 

 scheme of the development of an enteron by the formation 

 of a cavity at the central meeting-point of a colony of amoe- 

 boid cells, and the subsequent differentiation of two cell- 

 layers already foreshadowed in the differentiation of the outer 

 and inner portion of each cell. The hypothesis of precocious 

 segregation explains the common replacement of the original 

 process of delamination by invagination, and accounts for the 

 blastopore. The blastopore being thus explained, we have no 

 further assumptions to make as to primary and secondary 

 mouths, and we avoid the fatal objection Avhichcan be urged 

 against HaeckeFs theory — that it admits of a reductio ad 

 absurdum, since, in reasoning from it, we are driven to the 

 conclusion that the mouth of a whelk is the homologue of 

 the anus of a water-snail. 



II. Formation of the mesoderm and body-cavity (c(elom). 



So far I have only discussed the origin of the two primitive 

 cell-layers and the primitive dominant organ of the animal 

 economy — the enteron. External changes of shape and 

 prolongations of cells (cilia) or groups of cells (tentacles), as 

 locomotor and prehensile organs, have not been touched 

 upon. These I shall allude to in due course, but first of all 

 must deal with important changes which supervene in regard 



