This paper presents a study on thinking and learning processes of mathematics and science in teaching through a foreign language, in Finland. The entity of thinking and content learning processes is, in this study, considered as cognitional development. Teaching through a foreign language is here called Content and Language Integrated Learning or CLIL. CLIL refers to all those diverse programmes, including some forms of immersion and bilingual education, where a foreign language is a medium of instruction, affecting the entire learning process of the learner. Thinking and content learning in CLIL manifests itself as analogical CLIL reasoning systems, based on two languages, and is assumed to affect cognitional development. Cognitional development was studied with 669 Finnish mainstream L1 learners aged 7-15 in the public comprehensive school. The experimental group, 335 learners, was taught through English, French or Swedish. The experimental group was compared with a control group of 334 learners, taught through the mother tongue, i.e. Finnish. Cognitional development was studied in terms of individual concepts and conceptual structures that are here called meaning schemes. The results of four measurements in 2002-2003 are presented in which statistical differences were found between the experimental and the control group in cognitional development.