198 ESSENTIALS OF BIOLOGY 



cells of the lens and cornea ; connective tissue cells ; pigment 

 cells, etc. This is the process of histogenesis that continues 

 that of organogenesis. It may be regarded as complete in the 

 cases where development pauses and a viable larva results — as in 

 the case, for instance, of the Nauplius larva of a Barnacle. When 

 organogenesis is resumed, upon the transformation of the Nauplius 

 into the Cypris-larva histogenesis is also resumed. In the cases 

 of continuous development the histogenesis is seldom complete 

 when the foetus is born. Thus the differentiation of nervous 

 tracts in the brain and spinal cord of the human infant is not 

 completed until at least a year after birth. 



The processes whereby embryonic cells become transformed 

 into tissue-elements have been described in many cases of develop- 

 ment. But obviously these descriptions are not necessarily 

 explanations. How the cells in one part of an organ-anlage 

 become muscle-cells while those in closely adjacent parts become, 

 say, glandular cells is just as fundamental and unsolved a problem 

 as that which is involved in the tectonic arrangement of the 

 embryonic cells of the organ-anlage. 



71 fl. De-differentiation. As a very general rule both the 

 processes of organogenesis and histogenesis are irreversible. 

 But there are cases of development where the tectonics of the 

 organs break down and the structure of the tissue-cells reverts 

 to the embryonic, or undifferentiated type. This is the case 

 with the Cirripede parasite, Sacculina : in the phase of the 

 Cypris-larva the animal is completely developed and is able to 

 lead an independent life in the sea (though it does not reproduce). 

 When it attaches itself to its host (a crab) its organs and tissues 

 lose their structure and the body apparently becomes a small mass 

 of embryonic, or undifferentiated cells, which then enter into the 

 body of the host. Somewhat similar phenomena are observed 

 when the head of the Hydrozoon, Tiibulariay is cut off and when 

 a new head regenerates from the tissues of the stalk. First the 

 cells of the latter part become de- differentiated and then a new 

 process of organogeny and histogenesis begins. The same 

 reversal of normal development may be seen in the growth of a 

 malignant tumour, in, say a mammalian animal. The most 

 striking thing in tumour growth is the very rapid divisions of the 

 cells of the organ that takes on malignant characters, but more 

 significant perhaps is the reversion to embryonic type, or 



