EXPLANATION OF PLATE. 



The circles (O), eights (8), and thetas {S) represent in each ease the nucleus or 

 nuclei belonging to a cell, and the succession of cell generations is shown by a 

 string of nuclei either simple, in pairs (apaylogamic douV)le cells), or fused (para- 

 gamic double cells). The half circles (C^^) and (puidrants ( 03 d ) represent the two 

 cell generations formed during chromosome reduction. The l)rackets [ ] represent 

 a cell at tlie period or periods when the organism is reduced to a unicellular condi- 

 tion. All the sighs for nuclei could he supposed to be inclosed by a cell wall, which 

 has been omitted for the sake of clearness. For the same reason only the cell lineage 

 leading directly up to the formation of the gametes has been shown, and no account 

 has been taken of the enormous multiplication of cells which occurs not only to build 

 up the bodies of animals and plants but also to form many gametes. Only a few of 

 the numerous cell generations which make u}) the organisms in question are shown. 



EXPLANATION OF SIGNS. 



p Plasmapsis — fusion of the cytoplasm, or unspecialized protoplasm. 



K Karyapsis — fusion of the nuclei, or nuclear protoplasm. 



(§) Synapsis — fusion of the chromatin elements. 



Heterotypic and homotypic divisions following synapsis. 



Nuclei of haplogamic phase — structures composed of simple cells having 

 OOOO nuclei and chromatin elements completely fused. 



QQQQ Nuclei of apaylogamic phase — structures composed of double cells, each 



0000 having two unfused nuclei. 



Nuclei of paragamic ])hase— structures composed of double cells having 

 wc3© single nuclei containing unfused chromosomes. 



[ ] Cell, at periods where the organism is reduced to a single cell. 



1 I The expanded egg. 



EXPLANATION OF FIGURES. 



Fig. 1. — Lower organism, such as 8ph;eroplea, having only simple-celled (haplo- 

 gamic) tissues. The fertilized egg undergoes no development beyond merely splitting 

 up into four spores when it germinates. 



Fig. 2. — Higher fungus, such as Agarieus or Puccinia, showing alternation of simple- 

 celled and double-celled phases, the latter apaylogamic, i. e., with two unfused 

 parental nuclei in each cell. The fertilized egg has expanded into a mass of apay- 

 logamic tissue. 



Fig. 3. — Moss, showing alternation of a long simple-celled and a short double-celled 

 phase, the latter paragamic, i. e., with parental nuclei fused but with their chro- 

 mosomes still distinct and unfused. The fertilized egg has expanded slightly into a 

 mass of paragamic tissue. 



Fig. 4. — Fern, showing alternating phases as in moss (figure 3), but with a short 

 simple-celled phase and a long double-celled phase, the paragamic phase having 

 developed at the expense of the Viaplogamic. The fertilized egg has expanded very 

 much into a mass of paragamic tissue. 



Fk;. 5. — Flowering plant (phanerogam), showingalternationof a very short simple- 

 celled phase with a very long double-celled phase, the paragamic phase having 

 developed greatly at the e^cpense of the haplogamic. The egg mother-cell develops 

 only one cell (macrospore). The fertilized egg expands into a large mass of para- 

 gamic tissue in which the greatly reduced haplogamic phase develops in a semipara- 

 sitic manner, it having no free existence. 



Fig. 6. — Higher animal, having only double-celled tissues, the haplogamic phase 

 having been completely suppressed by the greatly expanded paragamic phase. The 

 egg mother-cell develops only one egg. The fertilized egg has expanded into a large 

 mass of tissue. 



o 



26 



